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Optimization of zirconia-based pastes for additive
manufacturing of dental prosthesis
Madalena Afonso Vilar de Castro Paredes
Thesis to obtain the Master of Science Degree in
Bioengineering and Nanosystems
Supervisor: Doctor Ana Paula Valagão Amadeu do Serro
Supervisor: Doctor Célio Gabriel Figueiredo-Pina
Examination Committee
Chairperson: Doctor Gabriel António Amaro Monteiro
Supervisor: Doctor Célio Gabriel Figueiredo-Pina
Members of the Committee: Doctor Ana Mafalda Saldanha Guedes
December 2017
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Acknowlegments
This master thesis was only possible due to some people’s contribution, who directly or indirectly helped
me on this project. I would like to express my gratitude for their support throughout this journey.
First of all, I am deeply grateful to my supervisors Doctor Ana Paula Serro and Doctor Célio Figueiredo-
Pina, who had always showed an extraordinary availability and patience. I would like to thank both for
the encouragement and for being tireless along these months. Also, I would like to thank for giving me
this opportunity, which has contributed for my professional growth.
To Doctor José Mata, I would like to thank for his availability and help when it was needed.
I would like to thank to Professor Anabela Raymundo for all the availability with the rheometer and for
helping me whenever I needed to. To my college Rita Faustino and her supervisors, for giving me access
to CTN and for all the assistance. To Doctor Rogério Colaço and Doctor Mário Polido for the availability
on some devices.
For all the support and wisdom, I would like to thank to Engineer Marco Medeiros.
My gratitude goes also to all my laboratory colleagues, Diana Silva, Ana Topete, Diogo Silva, Mariana
Freire and Sofia Oliveira for all the help, integration and good disposition.
A special thanks goes to Ana Catarina Branco, for her friendship, encouragement and motivation. Thank
you for helping me whenever I needed to. Without you, this journey would not have been the same.
Finally, a huge thanks to my generous family, especially to my parents and my sister who made all my
accomplishments possible, for their exceptional support and unconditional love. Everything I've
achieved I owe it to you.
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Abstract
Dental restorations are often used to solve function and aesthetic problems of native teeth. As
restorative materials, ceramics became increasingly popular because of their aesthetics, inertness,
mechanical properties and biocompatibility. Additive manufacturing, namely robocasting, of ceramic
materials has gained significant interest to produce customized pieces, leading to low time of processing
and materials consumption.
The main objective of this work was the optimization of zirconia-based pastes to produce dental
prosthesis through the robocasting 3D technology. The rheology of several pastes (zirconia-based
pastes, containing the dispersant Dolapix CE64 and an additivative zirconia-based paste supplied by
CTCV) was studied, concerning the amount of dispersant, the method of preparation and the solids
content. Density and porosity measurements, as well as preliminar printability tests, were carried out. In
addition, topography/morphology, wettability, tribological behavior, microhardness and toughness were
also evaluated and compared with commercial zirconia.
Overall, taking into account the preliminary test of printability, the results showed that specimens
produced with paste containing 92% solids and 1% Dolapix may be used for production of dental parts
by robocasting, since the produced specimens present adequate properties for dental applications, with
roughness values of 6.53 nm, microhardness values above 1100HV and toughness values between 3–
6.5 Pa⋅m1/2.
Key words: Additive manufacturing, robocasting, dental restorations, zirconia pastes, materials
characterization.
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Resumo
As restaurações dentárias são frequentemente usadas para resolver problemas funcionais e estéticos
de dentes nativos. Como materiais restauradores, a cerâmica tornou-se cada vez mais popular devido
à sua estética, inércia, propriedades mecânicas e biocompatibilidade. O fabrico de aditivos de materiais
cerâmicos, nomeadamente por robocasting, ganhou interesse significativo para produzir peças
personalizadas, resultando num reduzido tempo de processamento e consumo de materiais.
O principal objetivo deste trabalho foi a otimização de pastas à base de zircónia para produzir próteses
dentárias através da tecnologia de impressão 3D por robocasting. Estudou-se a reologia de várias
pastas (pastas à base de zircónia, contendo o dispersante Dolapix CE64 e uma pasta aditivada à base
de zircónia fornecida pelo CTCV), quanto à quantidade de dispersante, ao método de preparação e ao
conteúdo de sólidos. Foram realizadas medidas de densidade e porosidade, bem como testes
preliminares de printabilidade. Além disso, a topografia/morfologia, molhabilidade, comportamento
tribológico, microdureza e tenacidade também foram avaliadas e comparadas com zircónia comercial.
No geral, tendo em consideração o teste preliminar de imprimibilidade, os resultados mostraram que
os espécimes produzidos com pasta contendo 92% de sólidos e 1% de Dolapix podem ser usados para
a produção de peças dentárias por robocasting, uma vez que os mesmos apresentam propriedades
adequadas para aplicações dentárias, com valores de rugosidade de 6.53 nm, valores de microdureza
acima de 1100HV e valores de tenacidade entre 3–6.5 Pa⋅m1/2.
Palavras-chave: Manufactura aditiva, robocasting, restaurações dentárias, pastas de zircónia,
caracterização de materiais.
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Table of Contents
Acknowlegments .................................................................................................................................... i
Abstract .................................................................................................................................................. ii
Resumo.................................................................................................................................................. iii
List of Figures ....................................................................................................................................... vi
List of Tables ........................................................................................................................................ ix
List of Abbreviations ............................................................................................................................. x
Chapter 1 – Theoretical Introduction ................................................................................................... 1
1.1 Human teeth ..................................................................................................................................... 2
1.1.1 Tooth microstructure .............................................................................................................2
1.1.2 Mechanical and tribological behaviour of natural tooth ..........................................................3
1.1.2.1 Friction and Wear .............................................................................................................4
1.1.2.2 Wettability/Oral Environment ............................................................................................5
1.2 Artifical dental materials ................................................................................................................. 6
1.2.1 Ceramic materials in prosthodontics .......................................................................................6
1.2.1.1 Zirconia-based ceramic ....................................................................................................8
1.2.1.1.1 Problems related to zirconia restorations ..................................................................... 10
1.3 Dental materials processing for restoration ............................................................................... 11
1.3.1 Subtractive Manufacturing .................................................................................................... 11
1.3.2 Additive Manufacturing ......................................................................................................... 12
1.3.2.1 Robocasting ................................................................................................................... 15
1.3.2.1.1 Essential parameters for the robocasting materials ...................................................... 17
1.3.2.1.2 Rheological behavior ................................................................................................... 17
1.3.2.1.3 Solid contents in the ceramic powder suspensions ...................................................... 18
1.3.2.1.4 Addition of dispersants and other additives .................................................................. 19
Chapter 2 – Experimental Procedures .............................................................................................. 22
2.1 Materials ......................................................................................................................................... 22
2.2 Methods .......................................................................................................................................... 24
2.2.1 Preparation of suspensions .................................................................................................. 24
2.2.2 Rheological measurements .................................................................................................. 25
2.2.3 Preparation of the specimens ............................................................................................... 26
2.2.4 Samples characterization ...................................................................................................... 28
2.2.4.1 Density measurements................................................................................................... 28
2.2.4.2 Porosity analysis ............................................................................................................ 29
2.2.4.3 Surface morphology/topography ..................................................................................... 29
2.2.4.4 Wettability tests .............................................................................................................. 31
v
2.2.4.5 Tribological behavior ...................................................................................................... 32
2.2.4.6 Microhardness tests ....................................................................................................... 35
Chapter 3 – Results and Discussion ................................................................................................. 37
3.1 Characterization of the pastes ..................................................................................................... 37
3.1.1 Paste A ................................................................................................................................ 37
3.1.1.1 Effect of the dispersant content ...................................................................................... 37
3.1.1.2 Effect of the preparation methodology ............................................................................ 42
3.1.1.3 Effect of the solids content ............................................................................................. 44
3.2 Production of the specimens ....................................................................................................... 47
3.4 Caracterization of the specimens ................................................................................................ 49
3.4.1 Density measurements ......................................................................................................... 49
3.4.2 Porosity analysis .................................................................................................................. 49
3.4.4 Surface Morphology/Topography .......................................................................................... 52
3.4.3 Wettability tests .................................................................................................................... 56
3.4.5 Tribological behavior............................................................................................................. 58
3.4.6 Microhardness tests ............................................................................................................. 59
Chapter 4 – Conclusions .................................................................................................................... 62
Chapter 5 – Future work ..................................................................................................................... 63
References ........................................................................................................................................... 64
vi
List of Figures
Figure 1. Schematic sectional view of an human healthy tooth. [4] .......................................................2
Figure 2. Schematic diagram of the tooth with important interfaces. [8] ................................................3
Figure 3. Summary of clinical wear mechanisms. Adapted from [16] ....................................................5
Figure 4. Types of dental crowns. [24] .................................................................................................6
Figure 5. Literature values for fracture force (not under exact same conditions). Adapted from [26] .....7
Figure 6. (a–c) The three major polymorphs of zirconia, and the two alternative means by which
metastable tetragonal phase can transform to monoclinic phase. (d) Phase transformation toughening
and (e, f) aging. Adapted from [27] ......................................................................................................8
Figure 7. The various states of zirconia. [22] ..................................................................................... 10
Figure 8. Example of different routes to produce customized implants. [36] ....................................... 12
Figure 9. Subtractive manufacturing (A) versus additive manufacturing (B). [36] ................................ 12
Figure 10. 3-D Printing process. [37] ................................................................................................. 13
Figure 11. (A) Investment casting model in resin produced by Envisiontec. (B) Dental restorations
made using the new Concept Laser Mlab cusing metal laser melting system. (Adapted from [32]) ..... 15
Figure 12. Schematic diagram of robocasting (direct ink writing). A material dispenser connected to a
computer-controlled robot, scans across the building platform, depositing the ink material in a layer-by-
layer manner. (Adapted from [37], [42]) ............................................................................................. 16
Figure 13. (1) Graph of the shear stress as a function of the shear rate. (2) Graph of the viscosity as a
function of the shear rate................................................................................................................... 18
Figure 14. Scheme showing typical viscosity versus volume percent solids behavior for dispersed
alumina suspensions. For ideal robocasting, work close to the dilatant transition. [44] ....................... 19
Figure 15. Yttria Zirconia powder sample (left) and Dispersant Dolapix CE64 (right). ......................... 22
Figure 16. Chemical structure of the dispersant dolapix ce64. ........................................................... 23
Figure 17. Haake Mars Modular Advanced Rheometer System. [57] ................................................. 25
Figure 18. Rheological measurements of zirconia suspension on Haake Mars Modular Advanced
Rheometer System, located at Instituto superior de agronomia. ........................................................ 26
Figure 19. Silicone molds. ................................................................................................................. 27
Figure 20. (A) Optimized paste; (B) Paste B and (C) Comercial samples ........................................... 27
Figure 21. Resin phenolic green (Right) and one of the obtained samples. ........................................ 27
Figure 22. Sartorius YDK03 Balance with copled support for measurement of apparent mass ........... 29
Figure 24. AFM Operational modes. Adapted from [60] ..................................................................... 30
Figure 25. Interaction between a liquid droplet and a solid surface, on a hydrophilic surface. [64] ...... 31
Figure 26. Tribological test on the zirconia commercial sample, regarding spherical-plane contact.
Adapted from [66] ............................................................................................................................. 34
Figure 26. square-based sharp pyramidal diamond indenter having specified face angles used on
Vickers hardness. [29]....................................................................................................................... 35
Figure 27. Indentation fracture. Adapted from [71] ............................................................................. 36
Figure 28. Viscosity as a function of shear rate for different amounts of Dolapix in 60%solids. ........... 37
vii
Figure 29. Overlap potentials of neighbouring raw material particles. Adapted from [72] .................... 38
Figure 30. Viscosity in function of the amount of dispersant, for 60% solids. ...................................... 39
Figure 31. Logarithmic scale of viscosity as a function of shear rate, for 0.8% of dispersant............... 40
Figure 32. Average of the consistence for the different amounts of Dolapix. ...................................... 41
Figure 33. Average of the flow rate for the different amounts of Dolapix. ........................................... 41
Figure 34. Viscosity as a function of shear rate for 1% and 1.5% of Dolapix in 60%solids. A typical
value of shear rate for the extrusion of ceramic pastes by robocasting is pointed out (20 s-1). ............ 42
Figure 35. Heterogeneos paste resulting from directly mixing 90% solids. ......................................... 43
Figure 36. Homogeneous paste based on the evaporation of water. .................................................. 43
Figure 37. Viscosity as a function of the shear rate for different solids contents in pastes with 1%
Dolapix. ............................................................................................................................................ 44
Figure 38. Average of the consistency for the pastes with different solids loadings and 1% Dolapix. .. 45
Figure 39. Average of the flow rate for the pastes with different solids loadings and with 1% Dolapix. 45
Figure 40. Viscosity versus solids loading behavior for zirconia slurries at 1 wt% Dolapix, at three
different shear rates. ......................................................................................................................... 46
Figure 41. Viscosity as a function of the shear rate for Paste B. ......................................................... 46
Figure 42. Logarithmic scale of viscosity as a function of shear rate. ................................................. 47
Figure 43. Extrusion of Paste A. A sintered sample is shown on the red square. ............................... 48
Figure 44. Extrusion of Paste B. Sintered samples are also shown. ................................................... 48
Figure 45. (1–3) Paste A: 90%, 91% and 92%, respectively; (4) Paste B and (5) Commercial Zirconia.
......................................................................................................................................................... 50
Figure 46. Porosity determined for the specimens produced with Paste A (for 90, 91 and 92% of solids
loading), Paste B and commercial zirconia, C. ................................................................................... 51
Figure 47. Distribution of sizes of the pores. (1–3) Paste A: 90%, 91% and 92%, respectively and (4)
Paste B. ............................................................................................................................................ 51
Figure 48. SEM image of sample A. .................................................................................................. 52
Figure 49. SEM images of sample B. ................................................................................................ 53
Figure 50. SEM image of commercial zirconia (C). ............................................................................ 53
Figure 51. AFM image of the surface area of Sample A, and the respective 3D profile of the surface
sample. ............................................................................................................................................. 54
Figure 52. AFM image of the surface area of Sample B, and the respective 3D profile of the surface
sample. ............................................................................................................................................. 54
Figure 53. AFM images of the surface area of comercial zirconia sample, and the respective 3D profile
of the surface sample. ....................................................................................................................... 55
Figure 54. Zirconia mean roughness (Ra). ......................................................................................... 55
Figure 55. Contact angle for zirconia samples. .................................................................................. 56
Figure 56. Wenzel (a) and Cassie-Baxter (b) models. ........................................................................ 57
Figure 57. Friction coefficient for zirconia samples............................................................................. 58
Figure 58. Zirconia microhardness for each sample. .......................................................................... 59
Figure 59. SEM image of an indentation in the commercial sample (C). ............................................. 60
viii
Figure 60. Fracture toughness of samples A, B and C, with the use of different equations. ................ 61
ix
List of Tables
Table 1. Mechanical properties of enamel and dentine. [1], [7], [12] .....................................................4
Table 2. Mechanical properties of the most used ceramic materials in fixed prosthodontics. Adapted
from [22] .............................................................................................................................................7
Table 3. Restorative zirconia based materials used for CAD/CAM systems. [30]..................................9
Table 4. Mechanical properties of 3 types of zirconia used in dentistry. [12] .........................................9
Table 5. Summary of 3-D printing methods for different types of materials. [37] ................................. 14
Table 6. Typical Engineering Performance Data for Yttria Zirconia [55] .............................................. 22
Table 7. Characteristics of Dolapix CE 64 dispersant. ....................................................................... 23
Table 8. Composition of Paste B. ...................................................................................................... 23
Table 9. Raw materials of the Zirconia paste prepared for robocasting. ............................................. 24
Table 10. Recipe for the artificial saliva, provided by [12]. .................................................................. 33
Table 11. Young’s modulus and Poisson coefficient of zirconia. [55], [68] .......................................... 35
Table 12. Average relative density of specimens obtained with Paste A (with different content of
solids), Paste B and commercial yttria zirconia. ................................................................................. 49
x
List of Abbreviations
AM – Additive Manufacturing
2D – Two Dimensional
3D – Three Dimensional
DEJ – Dentine-ename junction
Y-TZP – Yttrium cation-doped Tetragonal polycrystalline zirconia
CAD/CAM – Computer Aided Design/ Computer Aided Manufacturing
ZTA – Zirconia-Toughened Alumina
Mg-PSZ – Magnesium cation-doped partially stabilized zirconia
SM – Substrative Manufacturing
CNC – Computer numerically controlled
LDM – Liquid Deposition Modeling
DIW – Direct Ink Writing
RP – Rapid Prototyping
CT – Computer tomography
STL – Stereolithography
CTCV – Centro Tecnológico de Cerâmico e do Vidro
AFM – Atomic Force Microscope
SEM – Scanning Electron Microscope
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Chapter 1 – Theoretical Introduction
Human teeth are not only an important masticatory organ, but are also closely associated with the
pronunciation and facial aesthetics of humans. Thus, teeth play an extremely vital role in our daily life.
With ageing, various pathological factors and traumas, dental lesions such as caries, partial or total loss
of tooth tissue will inevitably occur. As a result, artificial dental materials, e.g. crowns, have been
gradually developed and used to restore and treat lesions of human teeth. [1]
The recent introduction of additive manufacturing (AM) techniques in the dental field has marked a
turning point in the prosthesis production technology, from the traditional hand made to the innovative
automated approach. [2] This 3D technology has the advantage of being faster and more efficient and
leads to less waste material. The speed and ease of use of this process opens the possibility to be the
own dentist to fabricate custom dental crowns in his office. However, the use of this technique for
producing crowns is still quite incipient. One of the main reasons has to do with the need of materials
optimization, which should present appropriate characteristics to be processed through the 3D
technology and simultaneously mimic the natural tooth. Currently, metals and alloys, ceramics and
composite materials are the most widely used for dental restorations and implants [1], but only polymers
are being used in AM.
In order to develop new dental restorative materials, it is imperative to evaluate and understand the
structure-property relationships of the human tooth. [3] In fact, to restore the mechanical function of
tooth it is not only important to study its intrinsic properties, including the bulk structural behavior, but
also to understand the mechanisms by which tooth structures resist functional forces in the mouth. Thus,
both mechanical properties and microstructural features of teeth are important to perceive stress
dissipation in the tooth, for developing biomimetic restorative materials and for the execution of clinical
dental pieces.
The main objective of this work is the optimization of zirconia-based pastes to produce dental prosthesis
through 3D technology. The main challenge of the work will be the development of printable materials,
with adequate mechanical and structural properties that meet the demanding requirements for dental
crowns restoration. Suspensions of zirconia particles will be used to create pastes. Different production
parameters will be tested, as the suspension solid content, the amount of dispersive agent, or the effect
of the presence of other aditives. Several characterization techniques will be applied to study the
properties of the obtain materials and infer about the possibility of producing dental crowns with the
materials under study, namely measurements of porosity, hardness, wettability, tribological behaviour
and surface morphology.
2
1.1 Human teeth
Human teeth have the main functions of cutting, tearing and grinding food, functioning in this way as a
mechanical device during the chewing process. [3] They present two distinct parts, the crown and the
root, being attached in a strangulated intermediate portion called the colon. Figure 1 shows a schematic
diagram of a healthy tooth. These parts are composed by two anisotropic structures: a calcified one,
(comprising enamel and dentin) and a non calcified (the pulp). Thus, the two most important elements
of a tooth that determine its resistance to the severe conditions found in the mouth are the external
enamel and the subjacent dentine. [1]
Figure 1. Schematic sectional view of an human healthy tooth. [4]
1.1.1 Tooth microstructure
From advanced materials science perspective, a tooth is a functionally graded composite material with
mineralized matrix and organic reinforcement. [3], [4] Dentin and enamel are calcified layered structures
which are bonded by a functional graded dentine-enamel junction (DEJ) layer. [4]
The crowns of human teeth are covered by dental enamel [3], which is the hardest tissue in the human
body. Dentine is normally considered to be more elastic and soft. [1]
Enamel contains on average 95% inorganic substance, 4% water and 1% organic substances by weight.
Hydroxyapatite which is a crystalline calcium phosphate, is the main mineral constituent. Its
stoichiometric form is [Ca10(PO4)6OH2]. However, it is known that in the enamel the hydroxyapatite is
non-stoichiometric, being [(Ca, M)10(PO4, Y)6(OH, X)2]; where M = Mg2+, Na+, K+, Sr2+, Ba2+; Y
= CO32−, H2PO4
−, HPO42−, SO4
2− and X = F-, Cl- and CO32−. [5] Both enamel protein and water are more
abundant in inner enamel close to the dentine-enamel junction. Human teeth enamel consists of
aproximately 5 µm diameter rods encapsulated by aproximately 1 µm thick protein rich sheaths that are
arranged parallel in a direction perpendicular to the dentin-enamel junction from dentin to the outer
3
enamel surface. In the central part of the rod, crystallite plates are parallel to the rod axis while those
near the edge of the rod usually have an angle near 15-45° to the longitudinal axis of the rods. The rod
unit is the most important part for the understanding the microstructure and function of enamel. [6], [7]
Dentine, a hydrated biological composite mostly made of phosphoric apatite crystallites, is the main
supporting structure of the tooth and is the second hardest tissue in the body after enamel. In its
composition, it has about 70% inorganic material, 18% organic material and 12% water. [6] It is
considered an elastic and soft part of the tooth that contains dentinal tubules, which extend through its
entire thickness. The tubules are surrounded by highly mineralized cylinders of peritubular dentine in
the crown, which are, in turn, separated by intertubular dentin. [3]
Figure 2. Schematic diagram of the tooth with important interfaces. [8]
The dentin-enamel junction (DEJ), which is normally considered as the biological interfacial region
between the dentin and outer enamel coating in teeth [9], is a unique junction between highly mineralized
tissues of different physical properties, embryogenic origins and matrix composition. [4]
1.1.2 Mechanical and tribological behaviour of natural tooth
Knowledge of the mechanical properties of human teeth is of utmost importance when considering
clinical treatment. It also guides the development of "tooth-like" restorers that mimic the structure and
properties of teeth. [10]
It has been experimentally verified that both enamel and dentin have a fragile behavior. The mode of
enamel fracture is highly anisotropic but, fot dentin it is less anisotropic. [10] In other words, the
anisotropic microstructure of the enamel, resultant from rod orientation and the presence of organic
components, controls the ability of its antifracture. An extensive bibliographic research reveals that
advanced characterization techniques were used to record the mechanical response of the different
anatomical parts of the human tooth. However, there is still a lack of correlation between basic
4
mechanical properties, such as hardness, and microstructural composition of various structural parts of
the human tooth. [3], [10] The main mechanical mechanical properties of enamel and dentine are
referred in Table 1. As already mentioned, enamel is the hardest tissue in the human body with a Knoop
hardess of about 250 – 500, equivalent to 5 times that of dentin (57–71 KHN). [11] Enamel is brittle and
easily broken if it loses the support of the flexible dentin below, so the combination of enamel and dentin
is very important to be able to withstand about a 20 MPa occlusion pressure approximately 3000 times
per day. Therefore, enamel must retain its shape as well as its fracture resistance and wear during load-
bearing function for the life of the individual. Understanding enamel fracture properties and its crack
propagation is crucial for both clinicians and materials scientists and helps in the clinical management
of tooth wear, which involves replacing missing dental tissue with dental materials, along with an attempt
to minimize causal factors. [6]
Table 1. Mechanical properties of enamel and dentine. [1], [7], [12]
Property Enamel Dentine
Young's modulus (GPa) 20.0 – 84.2 10.2 – 15.6
Shear modulus (GPa) 29 6.4 – 9.7
Tensile Strength (GPa) 0.030 – 0.035 0.040 – 0.276
Compressive Strength (GPa) 0.095 – 0.386 0.249 – 0.315
Shear Strength (GPa) 0.06 0.012 – 0.138
Knoop hardness 250 – 500 57 – 71
Density (Kg/m-3) 2500 2900
Another important property to be critically considered is the tribological behaviour, once oral
biomechanical functions can usually result in some relative movement between contacting surfaces that
can be teeth, restorations and implants. [3] Tribology is the science of the mechanisms of friction,
lubrication and wear of interacting surfaces that are in relative motion. [1] Basically, the tribological
response, being a system dependent property for a given tribocouple, depends on the mechanical
properties of both coupling solids (e.g., E-modulus, hardness and toughness). The transmission of
forces, energy conversion and alteration of the mechanical and chemical properties results in an
eventual removal of material. Thus, understanding of wettability, friction and wear behavior of human
teeth helps in the development of new dental restorative materials. [3]
1.1.2.1 Friction and Wear
Regarding teeth, due to their location and biomechanical function in the chewing processes, tribological
movements can occur which cause friction and wear of the teeth. [1], [12] Friction is, by definition, the
sliding of one object or surface against another, whereas wear is a process that occurs whenever a
5
surface is exposed to another surface or chemically active substances [13], which can result in a
progressive removal of the surface material by mechanical or chemical action. [1] Tooth wear is a
multifactorial phenomenon involving the interplay of biological, mechanical and chemical factors. [1],
[14] which means that the etiology of tooth wear and restorations vary depending upon joint pathology,
occlusion, muscle tune, lubricants, individual dietary habits, and the type of restorative material used.
[15], [16] In this way, friction and wear may result from direct contact between the teeth and particles
during mastication or from pathological processes such as bruxism (which is is the action of grinding of
teeth without the presence of food), toothbrushing and others, whose scheme is represented in Figure
3.
Figure 3. Summary of clinical wear mechanisms. Adapted from [16]
1.1.2.2 Wettability/Oral Environment
The oral environment has great importance in dental tribology [1] because saliva provides protection
against physical and chemical wear. Saliva acts as a lubricant that provides a reduction in friction
between teeth and food, between teeth themselves and between teeth and the oral mucosa/tongue and
preserving, at the same time, deglutition, chewing and the faculty of speech. [12], [17] In turn, wettability,
which is a surface characteristic known to affect the biological response of teeth or dental prostheses,
is closely related to the oral environment. [18] In general, wettability is quantified by the contact angle,
which is the angle between the tangent line to a liquid drop’s surface at the threephase boundary and
the horizontal solid’s surface, which defines whether the material is hydrophilic or hydrophobic. The
contact angle for water on a solid surface is regarded as a natural phenomenon and mainly depends on
the chemical composition and the structure of materials. Although there is lack of relevant research
about enamel substrates, some studies have been reported [12], [19], [20], where the enamel surface
is considered hydrophilic.
6
In this way, due to the need for greater durability of teeth and dental restorative materials, its important
to take into consideration the surface wettability to control properties like adhesion, lubrication, friction,
wear resistance and biocompatibility. However, the wettability of surfaces can be strongly affected by
surface roughness in many different ways, so this influence can be very significant and must be taken
under consideration. [21] Considering that the roughness is related to the surface preparation, porosity
must be also taken into account. These surface characteristics are of clinical importance since a
manipulation of these variables might facilitate the prevention of dental disease by avoiding dental
plaque formation, which is a microbial plaque.
1.2 Artifical dental materials
Due to various pathological factors and traumas, patients may suffer dental injuries. To address this
problem, various artificial materials have been developed to be used in dentistry, in order to replace
missing teeth or non-esthetic but healthy tooth tissue and tooth enamel. Some examples are shown in
Figure 4. The practical goals of these treatments are based on the recreation not only of the function
but also of the aesthetics. To select a material for dental application, it is necessary to take into
consideration that the choice depends on several factors, such as corrosion behavior, mechanical
properties, including strength and wear, cost, availability, biocompatibility and aesthetic values. Usually
dental materials can be grouped into four categories: metals and their alloys, ceramics, polymers and
composites. [1] Despite the good long-term results, the moderate aesthetics and biocompatibility
concerns associated with the metals have increased the use of other materials. [22] Dental literature
indicates that ceramics are considered one of the most suitable materials to be used in dental
restorations. [23]
Figure 4. Types of dental crowns. [24]
1.2.1 Ceramic materials in prosthodontics
Within the dental field, ceramics offers advantages regarding their own nature and their chemical and
optical properties. Ceramic materials are highly resistant to acid and corrosion attacks and are therefore
regarded as exceptionally biocompatible. Furthermore, ceramics have a high resistance against wear
and, recently, due to the increasing aesthetic demand, the use of all-ceramic prostheses (like full-
coverage crown) has become popular [23]. Aesthetics is an obvious attribute of some ceramics and,
along with biocompatibility, durability and etchability (ability to be bonded), are the primary reasons why
dentists often choose ceramics over other materials. [1], [23].
7
However, there are relevant issues that arise in the dental use of such material. These problems are
related to its potential for catastrophic brittle fracture as well as its potential for abrasion of opposing
natural teeth or restorations, leading to a poor durability. In order to minimize damage from brittle
fracture, many efforts have been made in the last decades to develop high strength dental ceramics.
Feldsphatic porcelain, leucite reinforced felspathic porcelain, magnesia based porcelain, lithium
disilicate glass-ceramic, polycrystalline ceramics (as alumina and zirconia, which does not have glassy
matrix), and others, are some examples of these developed ceramics. [25] In addition, tetragonal
polycrystalline zirconia (Y-TZP), a high hardness zirconia ceramic, was developed as an alternative to
porcelain or glass ceramics in posterior restorations, raising particular interest [1] due to its great
resistance to fracture when compared to other materials. Figure 5 and Table 2 report several studies
made for fracture force, flexural strength and frature thoughness for some of these materials. In
addiction, zirconia’s biocompatibility has been proved both in vitro and in vivo as a non-cytotoxic and
non-mutagenic material, and no local or systemic adverse reactions have been identified. [22]
Figure 5. Literature values for fracture force (not under exact same conditions). Adapted from [26]
Table 2. Mechanical properties of the most used ceramic materials in fixed prosthodontics. Adapted
from [22]
Ceramic system Reinforcing material Flexural strength (MPa)
Fracture toughness (MPa/m1/2)
Glass-ceramics Feldsphatic porcelain 100 1.0
Glass-ceramics with fillers
Leucite 160 1.5 – 1.7
Lithium disilicate 300 – 400 2.8 – 3.5
Crystalline ceramics with glass fillers
Glass-infiltrated alumina 236 – 600 3.1 – 4.6
Glass-infiltrated zirconia toughened alumina
421 – 800 6 – 8
Polycrystalline ceramics Densely sintered alumina
487 – 699 4.48 – 5.6
Yttria stabilized zirconia 900 – 1200 9 – 10
659
770
950
1331
0 200 400 600 800 1000 1200 1400
Glass-Infiltrated Alumina
Alumina Strengthened with Zirconia
Lithium Dislicate Glass Ceramic
Zirconia Y-TZP
Fracture Force (N)
8
1.2.1.1 Zirconia-based ceramic
Zirconia, due to its excellent mechanical properties, holds a unique place amongst ceramic oxides. This
situation stems from the considerable amount of research work that has been conducted since the
discovery of the transformation toughening capabilities of zirconia in the mid-1970s. [27]
Under ambient pressure, the temperature influences the crystallographic form of unalloyed zirconia,
allowing it to assume three crystallographic forms: pure zirconia phase or monoclinic phase (m) (a
deformed prism with parallelepiped sides); tetragonal phase (t) (a straight prism with rectangular sides)
and cubic phase (c) (square sides). At room temperature and upon heating up to 1170 °C, the structure
is monoclinic. Then it is tetragonal between 1170 and 2370 °C and cubic above 2370 °C and up to the
melting point. [27] The best mechanical properties of zirconia are achieved only if some grains are able
to transform under stress from the tetragonal metastable state towards the stable monoclinic state,
concept which is called phase transformation toughening. In other words, zirconia can be used as a
structural bioceramic only if it is not completely stable. However, upon cooling, transformation from the
tetragonal (t) phase to the monoclinic (m) phase will induce a substantial increase in volume (∼4.5%),
which will lead to catastrophic failure. Even so, this necessary metastability in a moist environment,
may cause a spontaneous transformation from the metastable tetragonal phase to the stable monoclinic
phase, decreasing the strength of the material, phenomenon which is called aging (or low temperature
degradation) as shown in Figure 3. [22],[27] Researchers found that the addiction of CaO, MgO, Y2O3 or
CeO2 to zirconia-alloys allows the retention of the tetragonal structure at room temperature. This will
control the stress-inducing t→m transformation, which induces compressive stresses, thereby closing
the crack tip, preventing further crack propagation and leading to high toughness. [28]–[30] Also, the
addiction of stabilizers can prevent or decrease aging. [22] This unique characteristic gives zirconia
superior mechanical properties compared to other dental ceramics.
Figure 6. (a–c) The three major polymorphs of zirconia, and the two alternative means by which
metastable tetragonal phase can transform to monoclinic phase. (d) Phase transformation toughening
and (e, f) aging. Adapted from [27]
9
Thus, new materials and technologies to process zirconia based ceramics, such as CAD/CAM, have
been developed. [23] In vitro studies demonstrated a flexural strength of 900–1200MPa and a fracture
toughness of 9–10MPa.m1/2. [30] However, these parameters may change depending on the treatment
of the restorative material (exemplified in Figure 7), as shown for flexural strength in Table 3. [29]
Table 3. Restorative zirconia based materials used for CAD/CAM systems. [29]
Restorative material Flexural strength (MPa)
In-Ceram Zirconia (zirconium oxide) 750
Partially sintered zirconia (zirconium oxide) 800 – 1300
Full sintered zirconia (zirconium oxide) >1000
The recent introduction of ceramic-based zirconia as dental restorative materials has generated
considerable interest in the dental community. [28] Although there are several types of zirconia-
containing ceramic systems currently available, to date only three are used in dentistry. These are
yttrium cation-doped tetragonal zirconia polycrystals (3Y-TZP), zirconia-toughened alumina (ZTA) and
magnesium cation-doped partially stabilized zirconia (Mg-PSZ), whose properties are presented in
Table 4. [28]
Table 4. Mechanical properties of 3 types of zirconia used in dentistry. [12]
Property Y-TZP ZTA Mg-PSZ
Young Modulus (GPa) 210 380 210
Compressive Strength (GPa) 2 2
Density (Kg/m-3) 6000 4150 5850
Flexural strength (MPa) 950 600
Flexural strength (MPam1/2) 10.5 4–5 5.8
Vickers Hardness (HV0,5) 1250 1600 (HV30) 1250
By comparing the various parameters of the Table 4 and through some literatures [22],[26], it is possible
to conclude that, in general, stabilizing zirconia with yttrium oxide enhances its mechanical properties
more than with other oxides. Zirconia can be stabilized partially by 2 – 5 mol% of yttrium oxide or fully
by 8 mol% of yttrium oxide. Although, dental zirconia, is commonly partially stabilized. [22]
10
Figure 7. The various states of zirconia. [22]
1.2.1.1.1 Problems related to zirconia restorations
A few clinical problems related to zirconia restorations are known as secondary caries, marginal
discoloration, ceramic fractures, chipping of the veneering porcelain and loss of retention. [22] These
problems occur in various combinations to cause surface loss of materials in the mouth. Thus, problems
associated with restorations are based on crown microstructures, surface characteristics and
environmental influences.
Wear and Friction Coefficient
Wear properties of restorative dental materials differ from those of natural teeth and may alter the wear
rate of natural opposing teeth. So ideally, the wear resistance of the restorative material and the enamel
should be similar. [15] Although zirconia has a high resistance against wear, a possible abrasiveness of
this material restorations to antagonist teeth due to its hardness has raised concerns in dentistry. [22]
The applied load also has a significant effect on the friction coefficient and wear (i.e., if the surface of
the ceramic is already degraded, when subjected to occlusion, the dental wear process will be
accelerated).
Oral environment
Given the complex inter-oral environment and biomechanics, the degradation of the zirconia surface
can be due to the influence of pH from diet and intrinsic diseases. However, the natural conditions found
in the oral cavity (pH and temperature changes) are not severe enough to dissolve components from
11
this dental ceramic. [31] Low bacterial adhesion of the restoration is a crucial factor in the longevity of
the restoration. Crucial factors affecting dental plaque (which is biofilm or mass of bacteria that grows
on surfaces within the mouth) adhesion to the dental restoration are the surface roughness of the
material and, intimately related to it, the wettability. Less bacterial adhesion has been found on smooth
surfaces than on rough ones, and bacterial adhesion to ceramic materials like zirconia has been noted
to be low due to their surface characteristics. [22]
1.3 Dental materials processing for restoration
In dentistry, a constant search for new materials and new technologies that improve performance and
cost of dental restorations is carried out. [32] Nowadays, there are equipments, like intra-oral and extra-
oral scanners, that allow to reproduce the teeth/oral cavity dimensions, being possible to make dental
crown molds. However, the quality of intra-oral scanner is not yet satisfactory, because the digitizing
process is greatly affected by the clinical and physiological conditions, e.g. saliva, blood or patient
movements. In addition, the digitization of an oral impression is difficult due to its concave geometries
and it can lead to poor virtual representation. The indirect procedure, involving impression taking and
replica production, is more precise and therefore stone replica (i.e., plaster mold) is commonly digitized.
[2]
Therefore, restorations can be designed from a virtual model of the prepared tooth, using a specific CAD
tool. The availability of a geometric model for the restoration allows the production of a dental device
through subtractive or additive approaches as Computer Aided Manufacturing (CAM) or Additive
Manufacturing (AM) processes. This implies a demand for adaptations or development of materials that
follow the needs of the specific technology and the specific indication. [33]
1.3.1 Subtractive Manufacturing
The technology known as Subtrative Manufacturing (SM), is a computer numerically controlled
machining, which is based on processes in which power-driven machine tools, such as lathes, saws,
milling machines, and drill presses, are used with a sharp cutting tool to mechanically cut the material
(removing undesired materials) to achieve the desired forms. [33] Thus, subtractive manufacturing is a
process by which 3D objects are constructed by successively cutting material away from a solid block
of material (by conventional or unconventional machining), which forces designers to have in mind the
limitations of all manufacturing techniques involved on the product. Although it is a technology based on
CAD/CAM widely used, this method leads to a great waste because more material is removed compared
to what is used in the final product. It also involves tools wear, that require periodic substitution. In this
way, on a large scale, this results in relevant environmental problems. [34]
12
1.3.2 Additive Manufacturing
Additive manufacturing (AM) technology has been studied and developed for more than 20 years.
Instead of removing materials, AM processes are responsible for producing three-dimensional parts
directly from CAD models by adding materials layer by layer, which allows the building of parts with
geometric and material complexities that could not be reproduced by subtractive manufacturing
processes. [35] The integration of AM in traditional dental restorations manufacturing is an emerging
application. [2] In Figure 8 a flowchart illustrating the different routes in order to produce personalized
implants from polymers, metals or ceramics is shown.
Figure 8. Example of different routes to produce customized implants. [36]
Among the advantages of the additive process over conventional subtractive manufacturing methods is
the reduction of the raw material consumption. In fact, while in the subtractive manufacture a block of
material is processed by material removal machines, which the result will consequently lead to a large
amount of waste material, the volume of raw material used during the AM process is in practice close
to the volume of the part before the finishing phase. [33], [34] Other advantages are related with the
lower processing time, lower number of production steps and the adequacy to produce complex and
personalized pieces. Subtractive vs. additive manufacturing is shown in Figure 9.
Figure 9. Subtractive manufacturing (A) versus additive manufacturing (B). [36]
13
A significant advance has been observed in the development and commercialization of new and
innovative AM processes, also known as 3-D printing. [35], [37] As shown in Figure 10, the 3D printing
process starts when the object's digital model is obtained through CAD software, a 3D scanner or a
photogrammetry procedure. The design of the 3D model is then converted to a stereolithography file
(STL) format so a cutting process is applied using the printer software to convert the STL file data into
a G code file containing geometric information from a series of 2D layers in which the model is divided.
Finally, the printer begins to deposit the material following the layer-by-layer sequence dictated by the
G code file. [37] This technique has the main advantage of producing details such as undercuts, voids,
and complex internal geometries.
Figure 10. 3-D Printing process. [37]
Addictive manufacturing includes different processing categories. In each one, some technologies can
be highlighted, whose characterization is made by specific processing and applications. Some of these
3-D printing methods are listed in Table 5.
Photopolymerization refers to the layer by layer or an entire layer at a time solidification of a curable
fluid medium and provides exceptional dimensional tolerance, but cannot manufacture with multiple
materials. [38], [39]
In turn, powder bed process is an additive manufacturing technology often also referred to as layer
melting. Within this technology can be found several techniques, being one of the most important and
used selective laser sintering, where a laser sintering applies a laser energy source to sinter successive
powder layers. Although this method results in high quality components with negligible porosity, it is not
compatible with multiple materials. [38], [39]
14
On the other hand, lamination is the process of mechanical conformation that aims to change shape
(i.e., thickness), by mass deformation. In this case, the created objects are not so precise and does not
have such complex geometries compared to other techniques, which leads to the disadvantage of low
quality surface since the individual layers are cut by laser beams. [38]
Finally, the extrusion process generates a three-dimensional object by the deposition of a filament
(solidified if it is in FDM, or not solidified if it is DIW) from the movable deposition head, allowing low cost
manufacture within a reasonably short build time, and accommodating multiple materials like metals,
polymers, and ceramics, being a technique of great success and with a wide application. In addition to
the techniques presented, there is also the 3D-printing process of injecting material, which offers high
resolution and low polymerization shrinkage and also allows fabrication of objects with a smooth surface.
[40]
The choice of the appropriate method to use depends on the consideration of some factors, such as the
complexity of the objects to be created, the material used, the number of replicas required and, of
course, the cost. [37] The feasibility of these different AM processes, has recently have been
investigated for the production of ceramic pieces. [41]
Table 5. Summary of 3-D printing methods for different types of materials. [37]
Therefore, the current dental CAD/CAM systems is not the best choice for mass production of crowns
and bridges since only one part can undergo machining at once. [32] In this perspective, Additive
Manufacturing is a great opportunity for the dental industry in the production of custom parts specially
made for the patient, where robocasting of ceramic materials has room for improvement. Although some
challenges are not explored yet, there is already several of additive manufacturing technologies that can
be used in dentistry (some examples are shown in the Figure 11).
15
Figure 11. (A) Investment casting model in resin produced by Envisiontec. (B) Dental restorations
made using the new Concept Laser Mlab using metal laser melting system. Adapted from [32]
1.3.2.1 Robocasting
Robocasting, depicted in Table 5, is an extrusion filament-based 3D printing process that emerged as a
distinct AM over the years. [42], [43]
Robocasting is a rapid prototyping (RP) technique that allows freeform fabrication of dense ceramics
and composites [44], by assembling geometries in a layer-by-layer process. This technique uses
computer-controlled extrusion of colloidal pastes (also named slurries, gels, or inks) onto a flat substrate
without additional molds or tooling. Robocasting acts by strategically print material in order to evolve
three-dimensional (3D) structure instead of removing the unwanted material from a given stock by
machining. The printing of the layers has some specific instructions based on the 3D computer model
of the structure (e.g., assembled using computed tomography (CT) scans, point clouds from optical
scanning, or stereolithography (STL) files). When the 3D model is created, the software slices it in
stacked layers, designing then a sequence of print operations to build the layers one above the other.
This approach has had success in the assembly of various structures with applications in several areas
ranging from electronics to bone scaffolds. It is also a process that has some potential advantages over
CAD/CAM systems due to its ability to "print" with multiple materials at one time as well as create graded
structures. Its capability to spatially grade composition and/or microstructure (e.g., porosity) to meet
specific designs or needs, without requiring a previous mold confers another advantage over CAD/CAM
systems. [45]
The process starts with the robocasting ink, which is usually an aqueous paste of ceramic particles
inserted into a pattern for a syringe. The paste is extruded as a continuous filament at a controlled rate
through a nozzle attached to the syringe, as shown in Figure 12. The ink requirements demand that it
must flow through the nozzle with modest pressure, retain the desired shape after deposition, dry with
minimal shape change after printing, and sinter to high density.
16
Doubtless, and as previously mentioned, robocasting is a very flexible technique with regard to the range
of materials that can be printed. However, it is somewhat limited in the shapes it can form. The pieces
will always have step edges due to the layerwise nature and supports must be printed to allow significant
overhangs or large spanning elements. The size of the edges is defined by the radius of the printed
filament, typically being 100 – 250 μm, while other techniques such as stereolithography may reach
one-fifth that size. The real advantage of robocasting lies on the mechanical quality of the material
produced and the flexibility of the technique to print different materials, which allows internal morphology,
shape, distribution and connectivity to be controlled more accurately. [43], [45]
Figure 12. Schematic diagram of robocasting (direct ink writing). A material dispenser connected to a
computer-controlled robot, scans across the building platform, depositing the ink material in a layer-by-
layer manner. Adapted from [37], [42]
In this LDM technology for ceramic materials, highly charged colloidal slurries or suspensions are
generally formed by a mixture of ceramic powder, water and additives (which act as chemical modifiers).
This paste exits from an extruder head through an orifice, whose position is controlled by the material
being deposited in thin sequential layers on a base, which may or may not be heated. [38]
17
1.3.2.1.1 Essential parameters for the robocasting materials
Robocasting offers a range of different materials that may be printed, from bioactive ceramics such as
hydroxyapatite and β-tricalcium phosphate to dental ceramics such as aluminum oxide (Al2O3) and
zirconium dioxide (ZrO2). [43]
The properties and composition of the paste are considered as the most important factors for
robocasting. The inks (or paste) should be free of air bubbles and homogeneous, contain a high volume
fraction of ceramic powder and flow properties suitable for extrusion, and still be capable of maintaining
their shape after printing. An ink should be highly shear thinning to allow extrusion through fine nozzles,
and also retain a degree of strength and stiffness to be self-supporting following printing. This stiffness
and strength equates to a large elastic component of the ink’s viscoelastic response. Aqueous inks are
also preferred due to their simplicity, lower toxicity, lower cost, and slower drying, while low
concentrations of organic are desired to allow rapid burnout and high densities. Various approaches
have been explored to satisfy each of these criteria, such as very high solids loading pastes which dry
during printing, polymer-solvent based inks dependent on solvent volatility, and colloidal ceramic
suspensions where the particles interact by van der Waals forces to form a weak network. [43]
Thus, for the success of the robocasting process in the production of a material, it is required a
synergistic control of:
• paste viscosity and rheology;
• percent solids in the ceramic powder suspension;
• dispensing rate of the slurry through the orifice;
• drying kinetics of the dispensed bead of slurry, which determine the optimum build parameters;
• computer code for optimal machine instructions.
When a proper balance of these variables is achieved, the robocasting can be used to make intricate
ceramic bodies that sinter in relatively strong, dense and defect free parts. [44]
1.3.2.1.2 Rheological behavior
Depending on their viscosity behavior as a function of shear rate, stress, deformation, and others, fluids
can be characterized into different categories. Fluids obeying Newton’s law where the value of viscosity
is constant are known as Newtonian fluids. In addition, if viscosity is constant the shear stress is linearly
dependent on velocity gradient. In turn, fluids in which the value of viscosity is not constant are known
as non-Newtonian fluids (e.g., colloidal suspensions of particles with flexibility and shapes that can form
temporary bonds between them). These categories are based on the relationship between shear stress
and the velocity gradient (rate of shear strain) in the fluid. These relationships can be seen in the figure
below for some categories. [46]
18
Figure 13. (1) Graph of the shear stress as a function of the shear rate. (2) Graph of the viscosity as a
function of the shear rate.
Pseudoplastic fluids, those whose apparent viscosity decreases with increased stress are particularly
interesting for robocasting since they have the ability to flow through small holes at modest shear rates.
Unlike other free-form manufacturing techniques, robocasting does not require organic polymerization
reactions or solidification of a polymer melt for solid transformation. Robocasting is based on the
rheology of the deposited pulp and the partial drying of the individual layers, so that it is possible to
maintain the structural integrity while constructing a component.
Summarily, a robocasting ink must satisfy the following criteria [44]:
• It must be pseudoplastic enough to be easily processed;
• After dispensing, it must be mounted in a non-fluid mass;
• It must be able to "accept" multiple layers without defects to form a uniform mass.
1.3.2.1.3 Solid contents in the ceramic powder suspensions
Typical ceramic powder suspensions have a relatively monosized distribution and their average particle
size is in the range of several microns. These type of ceramic powders, which are dried from a dispersed
slurry, typically have a consolidated structure which is approximately 65% of the theoretical density. [44]
For this technique of robocasting, the character of flowable slurries with solids loading just below the
consolidated density is of extreme importance. Figure 14 depicts schematically the behavior of a typical
dispersed alumina powder slurry. [44] At low solids loadings, dispersed suspensions have very low
viscosity and are rheologically Newtonian. In the present case, at about 40% by volume of solids, even
though the viscosity is still relatively low, the slurries begin to exhibit a pseudoplasticity shear-thinning
behavior. As the solids content approaches 60% by volume, both inter-particle interactions and collisions
begin to become dominant. In this case, the viscosity begins to increase appreciably, and the rheological
19
behavior becomes highly shear thinning. At approximately 63% by volume solids, the mobility of the
particles becomes restricted and the slurry blocks in a dilating mass.
Figure 14. Scheme showing typical viscosity versus volume percent solids behavior for dispersed
alumina suspensions. For ideal robocasting, work close to the dilatant transition. [44]
The pieces are typically constructed on a heated platform between 40 and 60 degrees Celsius to aid
the pseudoplastic transition to dilatant, which in turn must closely follow the build speed in order to
maintain structural integrity for thick parts. The drying rate is still an element to be evaluated, in the
sense that if it is very slow, the pseudoplastic transition to dilatant is delayed and the accumulated weight
of several layers eventually exceeds the yield stress of the pseudoplastic layers, a condition that can
induce slumping and the creation of non-uniform walls. On the other hand, if the drying rate is too fast,
warping, cracking and delamination may occur. [43], [44], [47]
In this perspective, it is recommended to robocast with slurries with solids loadings close to the dilatant
transition, so that with a minimum drying a robocasted layer becomes structurally sound and a base
upon which more layers can be deposited. [44]
1.3.2.1.4 Addition of dispersants and other additives
The stabilization of a suspension with respect to flocculation is determined by the forces of attraction
and repulsion between the particles, which depends on their physicochemical characteristics and the
particle-solvent interface. This stability is achieved when the repulsive forces are large enough to
overcome the attraction forces. To obtain a stable suspension dispersants (deflocculants) are used,
which, when adsorbed by the particles, increase the magnitude of the repulsion.
Dispersion of colloidal particles, with sub-micron and nanometer size, is a prominent issue in many
industries, including ceramics processing. Dispersion is achieved by addition of certain reagents which
adsorb onto the particles thereby increasing the interparticle forces to overcome aggregation.
20
Manipulation of the interparticle interactions (solid-solid and solid-liquid interactions) may allow the
control the rheological behavior, providing an approach to optimize the suspension properties. [48]
In this perspective, the optimization has as prerequisite: a well dispersed slurry with high solids loading
and is a preferred route to obtain near net shape components with a homogeneous and defect free
structure. The use of polyelectrolytes, such as Disperx N40 [49] and Disperx A40 (sodium polyacrylate),
Dolapix C64 and Dolapix C75 (ammonium salt of polymethacrylic acid) [48]–[50], and others, to promote
the stabilization of suspensions against flocculation has been one of the most widely used alternatives
in the ceramic industry, although several variables may affect the surface chemistry of particles including
pH, type of dispersant, impurities, solvent, ionic strength and others. [48], [51] In addition, several
organic additives, can be added to the suspensions in order to optimize the printability. These additives,
which are all miscible with water, are commercial humectants that prevent clogging and drying, and are
chosen according to the material safety data sheets of several commercial inks. [49] Ethylene glycol,
ethanol, urea, C-Caprolactam, 1,6-Hexanediol, Poly(vinyl alcohol), and others, are examples of some
of these organic additives. [38], [49]
In this way, robocasting focuses generally on the extrusion of highly loading ceramic pastes which are
typically 50 – 65% by volume of ceramic powder, < 1% by volume of organic additives and 35 – 50% by
volume of volatile solvent (usually water), although dispersants may optionally be used. [44], [45]
However, other compositions with higher content of solids may be considered in specific cases, to
optimize the processing of the materials.
Zirconia suspensions studies
In recent years, solid free-forming of ceramics has been a subject of intensive research [49] and more
materials have been developed. Some materials characteristics should be taken into account regarding
restorative dental applications, such as mechanical resistance (mainly strength, wear and toughness)
and correspondence of optical properties such as shade and translucency to those of natural teeth. The
recent use of high strength zirconia in dentistry industry has increased the spectrum of applications of
this ceramic material, leading to the manufacture of ceramic crowns. [52] In addition, while in the
conventional method a block of zirconia is processed, in 3D printing a suspension of ZrO2 particles is
used. Understanding the response of zirconia suspensions to processing can lead to more reliable
methods of fabricating ceramics.
Colloidal suspension of zirconia particles by addition of suitable dispersants has also been studied
widely. [49], [53] Typical dispersants which have been shown to disperse zirconia include Tiron,
Aluminon, ammonium polyacrylate, polymethacrylic acid, polyethyleneimine, and diammonium citrate,
2-propenoic acid and a metalorganic polymer, Titron X-114, Triammonium citrate, Dispex A40, and
Dolapix CE 64. These dispersants adsorb on to the particles thereby increasing the surface charge and
hence the interparticle forces, which in turn leads to a stable dispersion. [48]
21
However, studies reported for robocasting of zirconia are rare. Erwin Peng et al. studied the robocasting
of dense yttria-stabilized zirconia structures, where a suspension with low solid YSZ content (<38 vol%)
was readily shaped into complex structure and sintered to dense part while retaining its original
morphology without any warping, crack or deformation. [47] Nelson R.F.A Silva et al. reported the
evolution of a CAD/CAM process where ceramic pastes of alumina and zirconia were deposited in a
layer-by-layer sequence to build up crown structures by robocasting. Alumina and ZrO2 (Y-TZP) powder
were dispersed in water, using ammonium polyacrylate. In this work, a viscosifying agent named
hydroxypropyl methylcellulose were also used, as well as a counter polyelectrolyte (polyethylenimine),
in order to gel the slurry. This project resulted in pieces with a rough surface finish, so they concluded
that it is necessary to improve the automation of nozzle alignment for accurate support material
deposition. [45]
As a challenging job of dental restoration production, this project involving the optimization of zirconia
pastes for robocasting is totally innovator.
22
Chapter 2 – Experimental Procedures
In this study, zirconia-based pastes for robocasting were developed and characterized, trying to meet
the requirements described previously.
2.1 Materials
Zirconia powder containing 5.5 wt% Yttria (3 mol%) was purchased from Zircomet (Lot No. 131201) as
a chemically precipitated white powder or granule. The specifications, according to the supplier, were
as follows: purity>94.0%, BET surface area of 10–20m2/g and particle size (d50) of 150 nm. Typical
engineering performance data is also listed in Table 6.
The commercial polyelectrolyte Dolapix CE64 was ordered from Zschimmer and Schwarz (Germany),
and was used as a dispersant. Its chemical structure is given in Figure 15 and other characteristics of
the polyelectrolyte are listed in Table 7. Deionized and distilled water was used in the study, and the pH
adjustments were done using NaOH.
Figure 15. Yttria Zirconia powder sample (left) and Dispersant Dolapix CE64 (right).
Table 6. Typical Engineering Performance Data for Yttria Zirconia [54]
Density 6.04 g/cm3
Porosity 0 %
Tensile Strength 500 MPa
Compressive Strength >2000 MPa
Young's Modulus of Elasticity 205 GPa
Hardness (Vickers Hv0.3) 1350 Kg/mm2
Maximum Use Temperature 1000 ºC
Fracture Toughness K¹C 8.0 MPam½
23
Figure 16. Chemical structure of the dispersant Dolapix CE64.
Table 7. Characteristics of Dolapix CE 64 dispersant.
Appearance Yellowish liquid
Active matter Approx. 65%
Solubility Water-miscible
Density (20°C) Approx. 1.2 g/cm3
pH (original) Approx. 7
Residue on ignition Max. 0.1%
A paste based on zirconia was also supplied by the partner Centro Tecnológico da Cerâmica e do Vidro
(CTCV), whose composition was achieved by extrapolating the optimization (made by a master thesis
student [38]) of an alumina-based paste. This paste will be referred to as Paste B or Sample B and its
composition is given in Table 8.
Table 8. Composition of Paste B.
Raw materials %wt Function of
added
materials
Paste B
Zirconia
80 – 82 wt%
PVA (polyvinyl acetate) 5(*) Plasticizer
Deionized water 2 Solvent
Citric Acid >1,<3 (**) Dispersant
Powdered Sugar 10 Binder
(*) solution with a concentration of 20%
(**) solution with a concentration of 10%
PVA was selected as a plasticizer since it was evidenced that it softens the mixture before it cures,
making it more workable without affecting the final properties of the final product. Citric acid was used
as a dispersant, since it is known as dispersant for aqueous alumina suspensions. [38] Powdered sugar
24
was also used, which performs two functions: binder and rheology modifier during the formation and
processing of ceramic pastes. [38], [55]
Commercial zirconia, which is a block of 5Y-TPZ containing 8.80 wt% Y2O3 (5 mol%), was purchased
from Ivoclar Vivadent. This material is commonly used to produce dental restorations by SM, so it was
used for comparison purposes.
2.2 Methods
2.2.1 Preparation of suspensions
Suspensions were prepared by mechanically mixing different proportions of zirconia powder in
deionized-distilled water. These suspensions were stirred for about 30 min with a magnetic stirrer, and
pH adjustments were accomplished with 0.2 M NaOH to obtain pH 8 (as reported in literature [48] for
this dispersant). A fixed solid loading of 60 wt% was firstly used. Once the amount of dispersant that
must be added (recommended by the manufacturer) ranges between 0.1% to 0.5% of the solids content
of the mixture, suspensions having different levels of Dolapix CE64 were prepared in a slightly wider
range, 0.1% to 2%, in order to determine the concentration at which a minimum viscosity is obtained. In
order to study the effect of dispersant, and as suggested in the literature [48], Dolapix was added after
the pH adjustments were done. Calculations of the amounts of Dolapix CE64 to be added were based
on density. Detailed information about the components and dispersant concentrations (relative to the
solids content in the suspension) used is reported in Table 9 and 10.
Table 9. Raw materials of the Zirconia paste prepared for robocasting.
Steps of slurry
preparation
Raw materials Function
of added
materials
Density
(g/cm3)
Zirconia
Paste
Suspension
Zirconia powder Ceramic 6.04
Deionized-distilled water Solvent 1.00
Dolapix CE4 Dispersant 1.2
After selecting the ideal concentration of dispersant, i.e., the one from which results the lowest viscosity,
the amount of dispersant was fixed and the content of solids increased. For this, the paste with 60 wt%
solids (which corresponds to 20 vol%) was concentrated by evaporation: it was kept at about 50 °C in
an oven, for different periods till 48h, in order to evaporate water content. Then, suspensions of 70%,
80%, 90%, 91% and 92% in solids were obtained, for which viscosity measurements were also
performed. It was not possible to obtain more than 92 wt% solids (which corresponds to 65 vol%) since,
from this value, the paste was no longer homogeneous, presenting solid parts.
25
At the same time, it was also prepared a suspension with 90% in solids content, directly from the
mechanical mixing of the components without involving evaporation of water.
The zirconia suspensions were left to rest at room temperature in closed vessels until the next day, to
perform viscosity measurements.
2.2.2 Rheological measurements
Rheological measurements were made to study if the pastes were adequate to be used in robocasting,
i.e., if they presented pseudoplastic behavior.
The rheological properties of the zirconia suspensions were measured using a Haake Mars Modular
Advanced Rheometer System of cone and plate (C35/2° Ti L). Liquid paraffin was deposited over the
system to ensure that the suspension did not undergo evaporation during measurement. During these
measurements, the suspension temperature was kept constant at 20 ± 0.1 °C by a thermostat. The
viscosity measurements were performed as a function of the shear rate at different concentrations of
the dispersant. A range of shear rates was employed (30 points from 0.1 to 150 s−1) and measurements
were made over a period of approximately 30 s at each shear rate. The measuring program was
repeated three to five times for each sample to check reproducibility.
In turn, the rheological properties of the paste B were measured using a parallel-plates system (PP20
sensor), applicable to materials with very high consistency. The previous parameters remained
constant.
Figure 17. Haake Mars Modular Advanced Rheometer System. [56]
26
Figure 18. Rheological measurements of zirconia suspension on Haake Mars Modular Advanced
Rheometer System, located at Instituto Superior de Agronomia.
2.2.3 Preparation of the specimens
Firstly, it was attempted to simulate the extrusion involved in the robocasting using a syringe with a
needle having an internal diameter of about 0.5 mm. No needle was used to Paste B (it was used a
syringe having an internal diameter of about 2 mm). Spiral shape samples with approximately 6 mm
diameter were produced and sintered. Sintering was carried out at a temperature of 1450 °C for 2h with
a heating speed rate of 5 ºC/min and cooling in the furnace. After sintering, some of the samples were
mounted and polished (as described below), to be observed under the optical microscope and to study
its porosity, while others remained unpolished and were used for density measurements.
Since for further characterization samples with a regular shape and planar surfaces were needed,
manual printing was used to fill silicon molds of rectangular plates, with the prepared pastes. To prepare
the molds, small Perspex® parallelepipeds (acrylic; 3x1.5x0.5 mm3) were placed in a petri dish and
silicon was poured over them. Silicone 4514 T51 was left to polymerize with the parallelepipeds
immersed, for about 24 hours. The parallelepipeds were removed from the mold, and the optimized
zirconia paste (A), as well as paste B were extruded into the molds. The external surface of the samples
was then gently flattened with a spreader and samples were left to dry at room temperature for more
than one week. After drying and before sintering, the specimens related to the paste B were subjected
to an intermediate process named debinding. This step was only necessary on B samples due to its
high content of additives. The debinding process consists in the removal of binders and other additives
with temperature (500 ºC, 2 h) in order to avoid cracks, distortions or contamination. The obtained
specimens were then sintered with the same thermocycle referred above.
27
Figure 19. Silicone molds.
Figure 20. (A) Optimized paste; (B) Paste B and (C) Comercial samples
For polishing the sintered samples were previously mounted on resin phenolic green using a Buehler
Simplimet 2000 Automatic Mounting Press. Then, the samples were polished with silicon carbide
sandpapers accompanied with water with the sequential order of particle size of P600, P800, P1000,
P2400, P4000. Some of them needed to be initially polished with P120 and P400, due to its irregular
surface. After, in order to obtain a smooth surface, a polishing with 6 μm, 3 μm and 1 μm, grit diamond
pastes was done.
Figure 21. Resin phenolic green and one of the obtained samples.
28
2.2.4 Samples characterization
The sintered samples of paste A (the optimized paste), paste B (provided by CTCV) and of commercial
zirconia (samples C) were characterized concerning different properties.
2.2.4.1 Density measurements
Density measurements were performed on sintered specimens using the Archimedes principle. In
Archimedes' principle the density of a solid body can be defined as the relation between the mass of the
solid body and the mass of an equal volume of liquid. The deduced formula from the principle of
Archimedes which was used is given by Equation 1. [57]
𝜌𝑜𝑏𝑗𝑒𝑐𝑡 = (𝑚𝐶
𝑚𝐶 − 𝑚𝐴𝑃) × 𝜌𝐿 (𝟏)
where 𝜌𝑜𝑏𝑗𝑒𝑐𝑡 is the density of the solid body, 𝜌𝐿 is the density of the liquid (g/cm3), which in this case
will be water at room temperature, and 𝑚𝐶 and 𝑚𝐴𝑃 are, respectively, the real mass and the apparent
mass of the of the solid body. The calculation of the apparent mass was done by immersing the
specimen in distilled water at 20 °C, using a support coupled to the balance, as shown in Figure 22.
In this way, it is possible to calculate the relative density (Equation 2), which is the ratio between the
density (mass per unit volume) of a substance and the density of a given reference material. In this case
it was used as reference the density of the nanostructured zirconia used to prepare the pastes, (6,04
g/cm3).
𝑑𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒(%) = (𝜌𝑜𝑏𝑗𝑒𝑐𝑡
𝜌𝑟𝑒𝑓) × 100 (𝟐)
The measurements were made on a Sartorius YDK03 balance, at Campus Tecnológico e Nuclear
(CTN), which includes the proper density determination kit. For each sample, five weighing tests were
performed outsider and inside water, so the densities used for each sample were based on the mean of
these five essays.
29
Figure 22. Sartorius YDK03 Balance with copled support for measurement of apparent mass
2.2.4.2 Porosity analysis
To evaluate the porosity of the samples, they were observed in the optical microscope after polishing.
Some photographs were taken with resolutions of 5000X, 10000X and 20000X, not only to quantify the
porosity but also to evaluate the distribution and size of the pores. This evaluation was done using the
software ImageJ, which is a Java-based image processing program.
2.2.4.3 Surface morphology/topography
Topography
Sample surface topography can be studied using an atomic force microscope (AFM). [58] Atomic force
Microscopy (AFM) is a versatile and powerful technique which allows the sample surface analysis,
providing a three-dimensional image of the sample surface topography as well as various types of
surface measurement data of a sample at the nanoscale. This technique has many advantages,
including capability of extremely high resolution, and is a non-destructive technique where samples need
to be minimally prepared (no need for sample treatment or coating). [7], [59]
The AFM principle is based on three main steps, surface sensing, detection and imaging, and involves
four main components, a probe - flexible cantilever with a sharp tip, a piezoelectric scanner, an optical
detection system and a feedback system. [7] In this method different types of interaction maps can be
obtained, since there are several types of tip-surface interactions. In addition, distinct types of maps
require different positioning of the tip: the tip may be scanned in contact with the surface (contact mode),
scanned without contact (non-contact mode) or in intermittent contact (tapping mode).
30
Figure 23. AFM Operational modes. Adapted from [59]
The operation mode chosen for the tests of this experimental work was the “contact-mode”, where the
tip is simply moved across the surface, allowing the measure of the surface roughness. The
topographical measurements of the zirconia samples surface were carried out using an atomic force
(AFM) Nanosurf Easyscan2, at Instituto Superior Técnico. Topographical images were obtained through
contact mode at room temperature with an applied force of 25 nN and at a scan rate of 1.2 Hz, using
silicon probes (PPP-CONTSAuD-10, force constant 0.01-1.87 N/m). The software WSxM 5.0 Develop
4.0 was used to process all scanned images data and analyse the surface roughness. All topographic
AFM images are presented in a color scale which represents the Z height.
One sample from each group A, B and C was analyzed. From those samples, up to three randomly
selected regions of interest were scanned in order to calculate the average roughness value Ra (nm).
Images of 20 µm2 x 20 µm2 were obtained. It was considered the total area of the images for the
calculation of the average surface roughness by exporting the data to Excel.
Morphology
Scanning electron microscopy (SEM) is a method for high-resolution imaging of surfaces which uses a
multifunctional and complex microscope that allows the analysis of microstructure morphology by
obtaining images with high magnification. [60] To obtain an SEM image, the incident electron beam is
scanned in a raster pattern across the specimen’s surface and the emitted electrons are detected for
each position in the scanned area by an electron detector. The intensity of the emitted electron signal is
displayed as brightness on a display monitor and in a digital image file. By synchronizing the position in
the image scan to that of the scan of the incident electron beam, the display represents the morphology
of the sample surface area. Then, the magnification of the image is the ratio of the image display size
to the sample area scanned by the electron beam. All samples need to be able to handle the low
pressure inside the vacuum chamber since the SEM column and sample chamber need to be at a
moderate vacuum to allow the electrons to travel freely from the electron beam source to the sample
and then to the detectors. [60], [61]
In the present work, SEM images were taken to analyze the samples surface before and after wear and
indentation tests. Specimen from each tested group was selected. The surface of each sample was
recorded photographically at different magnifications to record the cracks from indentations. Prior to the
SEM imaging, the samples were cleaned and prepared. For both imaging analyses the samples were
31
coated in a vacuum evaporator with a thin layer of gold in order to to increase the conductivity of the
samples surface and also to prevent charges accumulation. The SEM used was a Field Emission Gun
(FEG) SEM JEOL JSM-7001F. The SEM imaging and the gold deposition were done in the MicroLab,
Electron Microscopy Laboratory of IST.
2.2.4.4 Wettability tests
Because mouth has an aqueous environment, hydrophilicity is an important surface property, known to
considerably affect dental properties and performance. [62] In this way, wettability, which is a physical
phenomenon that is closely related to oral structures, presenting all relevance for dental prosthetic
materials in terms of surface characteristics in interaction with saliva, is an important parameter to
evaluate. The wettability of a material by a liquid is assessed by measuring the contact angle, which is
defined by the angle formed between the solid surface and the tangent to the liquid surface in contact
with solid, in the intersection between a liquid, a solid and a gas.
Young’s equation (Equation 3) describes mathematically the wettability phenomenon [63] and relates
the contact angle (𝜃), with the interfacial tension between solid and vapour (𝛾𝑠𝑣), the interfacial tension
between the solid and liquid (𝛾𝑠𝑙), and the interfacial tension between the liquid and vapour (𝛾𝑙𝑣).
𝛾𝑙𝑣 𝑐𝑜𝑠 = 𝛾𝑠𝑣 − 𝛾𝑠𝑙 (𝟑)
The surface is hydrophilic when it has a low contact angle (i.e., surface wetting is high). When 0° < θ <
90°, the liquid wets the solid surface. On the other hand, the surface is hydrophobic when it has a high
contact angle (above 90°). In this case, the surface wetting is poor, i.e., when 90° < θ < 180°, it is
considered that the liquid does not wet the solid surface. When θ = 180°, there is no adhesion between
the two phases (i.e., there is no wettability). [7], [63]
Figure 24. Interaction between a liquid droplet and a solid surface, on a hydrophilic surface. [63]
32
The wettability can be measured by various techniques, such as the sessile drop or captive bubble,
where the contact angle is measured with a goniometer. The difference between the two lies in the fact
that in the sessile drop method a drop of the liquid is placed on the top of the surface that is being
analyzed, while in the captive bubble technique an air bubble is inserted below the surface, which is
immersed in the liquid.
Under experimental context, the contact angle was measured by the sessile drop method using the
goniometer system in order to evaluate the hydrophilicity of the sintered samples. Before wettability test
was done, samples were cleaned with ethanol and dried in vacuum oven for about 5h. After, the zirconia
samples were placed on the support surface. Using a micrometer syringe, a drop of distilled water was
placed over the sample. The shape of the drop was visualized in several sequential images obtained by
a video camera (JAI CV-A50) connected to a microscope WildM3Z and to a frame grabber Data
translation DT3155, during a period of time of 1 minute and the experimental analysis was performed
with the Axisymmetric Drop Shape Analysis Profile software. In each sample, 5 to 10 drops were placed
(1drop=1essay), and for each type of sample resulted fourteen contact angles, from which an average
was made.
2.2.4.5 Tribological behavior
Teeth wear may result in inefficient contact of the teeth during mastication and may lead to bad function
and deterioration of the temporomandibular joint. [58] Dental surfaces are biotribological systems whose
performance may be affected various mechanisms of friction and wear. [64] In this way, it is important
to evaluate the tribological characteristics of the zirconia samples that will eventually replace the teeth.
The nanotribometer equipment was used in this work for that purpose. It includes three main elements,
two optical fibers connected to the piezoelectric sensor, a cantilever (force transducer) and the counter-
body that interacts with sample surface. During tribological tests, the counter-body is loaded into the
sample with a precise known force. The relative movement between the two interaction surfaces induces
deflection of the cantilever which is detected by the spring bars. The two optical fibers emit a beam of
infrared radiation that is reflected by the two perpendicular mirrors allowing the measurement of vertical
and lateral variations of the normal force and the frictional force, respectively. In this way, the system
provides the coefficient of friction value (i.e., frictional forces of relatively moving surfaces in contact).
For the operation of this test, several characteristics were defined for setting up the nanotribometer
through the Tribometry software (Microsoft Windows 2000/XP platforms) and handling the data.
In this work, a CSM nanotribometer was used, in order to obtain the friction coefficients of the produced
materials and the commercial sample, and also to try to study the wear behaviour of the different
samples. It was used as lubricant artificial saliva (prepared on the day through the recipe shown in Table
9, with pH=6.5). The counter-body used was a zirconia sphere of 3 mm diameter supplied by Technoball
(hardness of 1350 HV, elastic modulus of 205 GPa), in order to simulate a zirconia/zirconia system. All
33
tests were carried out at room temperature under a constant vertical applied load of 100 mN, 1300
cycles (approximately 10 minutes), sliding distance of 1 mm/cycle and frequency of 2.12 Hz. InstrumX
software (version 4.4) was used to acquire the raw friction test data that was subsequently processed
and analyzed.
Table 10. Recipe for the artificial saliva, provided by [12].
Reagent Concentration (g/L)
NaCl 0.600
KCl 0.720
CaCl2 0.166
KH2PO4 0.680
Na2HPO4 0.337
KSCN 0.060
NaHCO3 1.500
Furthermore, it is still relevant to analyze the contact between these two elastic solids. The relation of
the deformation of the material with a normal applied force (FN) in a spherical-plane contact was defined
by Hertz in 1882 [65], as being:
𝐹𝑁 =4
3𝐸∗. 𝑅
12⁄ . (𝑅 − 𝑑)
32⁄ (𝟒)
Where R is the ration of the sphere curvature, (𝑅 − 𝑑) is the maximum deformation in the contact area
and E* the elastic modulus which is dependent on the Young’s modulus (E1, E2) and Poisson
coefficients (v1, v2) of both materials in contact, which in this case is the same material. E* is given by:
[65]
𝐸∗ =𝐸1𝐸2
𝐸2(1 − 𝜐12) + 𝐸1(1 − 𝜐2
2) (𝟓)
Since the contact area, A, is given by [65]:
𝐴 = 𝜋. 𝑎2 = 𝜋. (3
4.𝐹𝑁 . 𝑅
𝐸∗)
23⁄
(𝟔)
34
Figure 25. Tribological test on the zirconia commercial sample, regarding spherical-plane contact.
Adapted from [65]
The maximum compressive contact pressure 𝑃𝑚𝑎𝑥 applied across the contact area as an elliptic
distribution in the surface is given by [65]:
𝑃𝑚𝑎𝑥 =3. 𝐹𝑁
2. 𝜋. 𝑎2= (
6. 𝐹𝑁. 𝐸∗2
𝜋3. 𝑅2 )
13⁄
(𝟕)
While the mean compressive pressure (P0) is equal to 2 3⁄ of 𝑃𝑚𝑎𝑥, the shear stress 𝜏𝑚𝑎𝑥 is given by
[46]:
𝜏𝑚𝑎𝑥 = 0.31𝑃𝑚𝑎𝑥 (𝟖)
Table 11 reports the Young’s modulus (E1=E2) and Poisson coefficients (
𝜐1 = 𝜐2) of zirconia, in order to calculate the contact elastic component (E*). As previously mentioned,
the normal force chosen for the tribological tests was 100 mN, so a value of 𝐹𝑁 = 0.1 𝑁 was used to
calculate the mean pressure applied across the contact area. The applied average compressive contact
pressure obtained by applying the Hertz equation for elastic contact was 319 MPa. According to the
literature [12],[66] this value is within the masticatory pressure (compressive pressure between 35 and
350 MPa, if it is considered an occlusal contact area of 2 mm2), confirming the possibility of using this
normal force in this system.
35
Table 11. Young’s modulus and Poisson coefficient of zirconia. [54], [67]
Yttria Zirconia data S.I
E1, E2 2.05 x 1011 Pa
𝝊𝟏, 𝝊𝟐 0.31
R 1.5 x 10-3 m
2.2.4.6 Microhardness tests
Vickers hardness test is a mechanical test in which an indenter (shown in Figure 26) is forced into the
test-piece surface under a defined force, held for a defined duration and removed. This method allows
to measure the indentation hardness value which is obtained by measuring the depth or area of
indentation left in the tested material. Thereby, the indentation diagonal lengths are measured, the mean
result calculated, and this value then employed to calculate a hardness number which is equivalent to
the mean force per actual unit area of indenter surface contacting the test surface. [29] Vickers Hardness
is given by Equation 9:
𝐻𝑉𝐹 = 1,8544 𝐹𝑑2⁄ (𝟗)
where 𝐻𝑉𝐹 is the Vickers hardness number at applied force F (express as the mass in Kg from which
F is derived), and where 𝑑 is the mean length of the diagonals of the indentation (expressed in mm).
[58]
Figure 26. Square-based sharp pyramidal diamond indenter having specified face angles used on
Vickers hardness. [29]
36
In this work, one sintered specimen of each group (A, B and C) were randomly selected to measure
Vickers hardness using an indentation tester (HSV-30 Shimadzu model, from Instituto Egas Moniz).
About five to six indentations were made on the polished surfaces using a loading mass of 1kg, 10kg
and 30kg during 15 seconds, in five widely separated locations. Mean and standard deviation values
were calculated. The values of average indentation diagonals were obtained from at least five readings
in each sample.
Also, the corresponding crack lengths were measured from SEM micrographs. Fracture toughness of
sintered materials was estimated using the three indentation methods described in [10] and [68], by
several Equations:
𝐾𝑐,𝑖 =𝐻
𝐸×
𝑃
𝑐3
2⁄ (𝟏𝟎)
𝐾𝑐,𝑖 = 0.016 (𝐸
𝐻)
12⁄
×𝑃
𝑐3
2⁄ (𝟏𝟏)
𝐾𝑐,𝑖 = 0.0726 ×𝑃
𝑐3
2⁄ (𝟏𝟐)
where 𝐾𝑐,𝑖 is the indentation fracture toughness (Pa⋅m1/2), H is the hardness in GPa, E is the Young
modulus in GPa, P is the indentation load (N) and 𝑐 is the median length (m) of the most severe
produced crack in the corresponding indentation mark. The Young modulus shall be approached taking
into account the porosity of the materials, i.e., a value of E=E0(1–1.9P+0.9P2). [69]
Figure 27. Indentation fracture. Adapted from [70]
37
Chapter 3 – Results and Discussion
3.1 Characterization of the pastes
Two different types of zirconia-based suspensions were studied in this work: one containing a dispersant
polyelectrolyte and that was target of an optimization procedure (Paste A), and another one prepared
by CTCV (Paste B) containing several additives.
In the first one, Dolapix was used as dispersant. Dolapix CE64 is a carbonic acid based polyelectrolyte
and, according the manufacturer, is particularly suitable for deflocculation, due to the interaction
between the bivalente functional groups of the additive and the surface charge of the ceramic particles.
The paste optimization process involved the study of the influence of the Dolapix content, preparation
methodology and solids content. The second one contained PVA, citric acid and powder sugar, which,
as referred in section 2.1, acted, respectively, as plasticizer, dispersant and binder.
3.1.1 Paste A
3.1.1.1 Effect of the dispersant content
For the first type of pastes studied, the viscosity was determined as a function of shear rate at different
concentrations of Dolapix (0.1 to 2%) for suspensions containing 60% solids, to optimize the amount
of Dolapix that shall be added to the mixture. The obtained results are presented in Figure 28.
Figure 28. Viscosity as a function of shear rate for different amounts of Dolapix in 60%solids.
0
0,5
1
1,5
2
2,5
3
0 5 10 15 20 25 30 35
Vis
cosi
ty (
Pa.
s)
Shear Rate (1/s)
0.1%
0.2%
0.4%
0.8%
1%
1.5%
2%
pH 8
60 wt% solids
Dispersant dose
38
It can be observed that the viscosity decreases exponentially as the shear rate increases, for any added
amount of dispersant. With 0.1% of Dolapix, the viscosity changes in an irregular way for low values of
shear rate (< 5 s-1), which proves that this concentration does not make the suspension to steadily flow,
still being unstable. At concentration 0.2 and 0.4% a notable change is noticed, and the curves become
more regular. The suspension is less viscous and flows more easily. From concentration of 0.8%
forward, the peaks corresponding to the initial resistance to flow are quite smaller and the suspension
starts to steadily flow. With the increasing of the concentration of Dolapix to 1% and 1.5%, viscosity
values became quite low, there being practically no resistance to the suspension to flow. With 2%, the
resistance to the suspension flow reappears.
Therefore, as the dispersant concentration increases till 1.5%, the flow resistance decreases. This can
be explained by the adsorption of the Dolapix to the particle surfaces, which creates an electric double
layer and provides electrosteric stabilization, considerably reducing the interactions between the
particles and preventing them from forming agglomerates or a rigid network. In other words, with the
addition of dispersant, the charge of the particles increases which keeps them dispersed. For the optimal
electrolyte concentration, the zeta potentials of the neighboring particles lead to an ideal equilibrium
between attractive and repulsive forces. For higher Dolapix concentration (2%), the viscosity increases
because the particles diffuse layer becomes thinner when the electrolyte concentration increases. When
the electrolyte concentrations become too high, the attractive forces predominate and the tendency to
agglomerate increases.
Figure 29. Overlap potentials of neighbouring raw material particles. Adapted from [71]
The obtained results indicate that the amount of Dolapix affects the flow properties of zirconia
suspensions and that a higher shear rate is required to make the suspension flow.
Based on the data of Figure 28, several different shear rates were chosen to plot viscosity of zirconia
suspension against the amount of dispersant. It is known that for the extrusion of ceramic pastes through
a nozzle (as happens in robocasting), the overall shear rate distribution is not homogeneous, being zero
in the center of the noozle and maximum at the noozle wall. Depending if the fluid is Newtonian or not,
39
a V or a U-shape shear rate distribution is observed, respectively, with values that depend on the nozzle
diameter and printing speed. For a Newtonian fluid, Peng et al. [47], [72] determined that for a printing
speed of 5 mm/s and a noozle diameter of 0.41 mm, the maximum shear rate is 31 s-1. Shear rate values
around 20 s-1 may be used as reference for the extrusion of ceramic pastes in robocasting. [73] Results
are shown in Figure 30. It can be observed that to obtain a deflocculated dispersion, with the minimum
viscosity, a content of dispersant between 0.8% and 1.5% must be used.
Figure 30. Viscosity in function of the amount of dispersant, for 60% solids.
In order to reinforce the data obtained from Figure 30 and reach the objective of discovering the
concentration of Dolapix that allows to obtain a more stable dispersion (i.e., a minimum of viscosity), it
was necessary to study its rheological behavior in terms of consistency and flow rate. According to
Newton's law, viscosity (η) is defined as the quotient between the shear stress (𝜏), expressed in Pascal
(Pa), and the shear rate (γ), expressed in s-1:
𝜂 =𝜏
𝛾 (𝟏𝟑)
In the case of non-Newtonian fluids or suspensions, the variation between the shear stress and the
shear rate is not linear and can be related through the following equation, usually referred to as the
power law:
𝜏 = 𝑘. 𝛾𝑛 (𝟏𝟒)
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 0,5 1 1,5 2 2,5
Vis
cosi
ty (
Pa.
s)
Dispersant Dolapix CE 64 (% of the solids content)
2.653 1/s
4.393 1/s
7.275 1/s
19.95 1/s
33.04 1/s
Flocculated Deflocculated
Shear rate
Flocculated
40
where 𝛾 is the deformation rate, 𝑘 is the consistence (Pa.sn) and 𝑛 is the flow rate. The exponent, 𝑛,
characterizes the behavior of the fluid which depends on the behavior of viscosity against shear rate. In
Newtonian fluids, the viscosity depends only on temperature and pressure, where n=1. When n>1, the
fluid has a dilatant behavior, that is, its viscosity increases with the shear rate, and if n<1, the fluid has
a pseudoplastic or reofluidificant behavior (both non-Newtonian). [38], [46]
Since the fluid has a non-Newtonian behavior, in particular pseudoplastic (since viscosity decreases
with shear rate), the power law is expressed in the general form of Newton's law (Equation 15), which
is the simplest and most used to translate the (apparent) viscosity of non-Newtonian fluids.
𝜂𝑎 = 𝑘0. 𝛾𝑛−1 (𝟏𝟓)
From the linearization of Equation 15 we obtain the logarithmic function (Equation 16), being possible
to determine the value of 𝑛 by the slope of the line and the 𝑘0 by the ordinate in the origin.
𝑙𝑜𝑔𝜂𝑎 = 𝑙𝑜𝑔𝑘0 + (𝑛 − 1)𝑙𝑜𝑔𝛾𝑎 (𝟏𝟔)
The curves of viscosity presented in Figure 30 were then transformed to logarithmic scale and simplified,
adjusting the power law only to the reofluidificant zone, through the line equation obtained as:
𝑦 = (𝑛 − 1)𝑥 + log(𝑘0) (𝟏𝟕)
Figure 31. Logarithmic scale of viscosity as a function of shear rate, for 0.8% of dispersant.
-3
-2,5
-2
-1,5
-1
-0,5
0
- 1 , 5 - 1 - 0 , 5 0 0 , 5 1 1 , 5 2 2 , 5
Log
Vis
cosi
ty
Log Shear Rate
0.80%
Linear (0.80%)
Reofuidificant
zone
41
Then, the effect of varying amounts of Dolapix on zirconia suspension consistency and flow rate is
represented in Figures 32 and 33, respectively.
Figure 32. Average of the consistence for the different amounts of Dolapix.
Figure 33. Average of the flow rate for the different amounts of Dolapix.
As can be seen from Figure 32, the consistency decreases as the concentration of Dolapix in the solution
is increased to 1.5%, but slightly increases for 2%. Although the minimum value of consistency was
observed at 1.5% (0.04 0.01 Pa.s), this concentration of Dolapix leads to a much higher flow rate of
0
0,5
1
1,5
2
2,5
0.1% 0.2% 0.4% 0.8% 1% 1.5% 2%
Co
nsi
sten
cy (
Pa.
sn)
Percentage of DolapixCE64 in the suspension
0
0,1
0,2
0,3
0,4
0,5
0,6
0.1% 0.2% 0.4% 0.8% 1% 1.5% 2%
Flo
w r
ate
Percentage of DolapixCE64 in the suspension
42
the suspension (Figure 33), with a great dispersion of results (0.23 ± 0.13). On the other hand, a
minimum value of the flow rate was observed for 1% of Dolapix (0.02 ± 0.02). Since a content of 1% led
to a value of consistency very close to the observed minimum, and for shear rates above 5 s-1 the
viscosity of this suspension is consistently lower than the observed for 1.5% (all the experiments for 1%
led to values lower than the observed for 1.5% – see typical curves in Figure 34, which is a magnification
of Figure 28), it was decided to use 1% of Dolapix as the basis for the preparation of the suspensions
of zirconia particles with higher solids content.
Figure 34. Viscosity as a function of shear rate for 1% and 1.5% of Dolapix in 60%solids. A typical
value of shear rate for the extrusion of ceramic pastes by robocasting is pointed out (20 s-1).
3.1.1.2 Effect of the preparation methodology
Two strategies were used to prepare the pastes with Dolapix: direct preparation with a high content of
solids (90%) and increase of the solids content by evaporation (pastes with 60% of solids were heated
to evaporate water and get mixtures with higher concentrations of solids). In both cases the previously
defined optimal amount of Dolapix was used (1%).
The preparation of a suspension directly by mixing deionized and distilled water with dispersant in a
large amount of solids resulted in a heterogeneous paste, filled with small aggregates (Figure 35). This
happened because the amount of water with dispersant added was significantly less than the amount
of zirconia powder ( 10% 𝑤𝑎𝑡𝑒𝑟 / 90% 𝑧𝑖𝑟𝑐𝑜𝑛𝑖𝑎 ), which causes Dolapix CE64 to be unable to
homogeneously coat all particles of the paste. The agglomerates formed, permanent or not, besides
influencing the rheology of the suspensions, can interfere in the packaging of the produced specimens
0
0,005
0,01
0,015
0,02
0 5 10 15 20 25 30
Vis
cosi
ty (
Pa.
s)
Shear Rate (1/s)
1%
1.5%
43
and, consequently, in their microstructure, making this paste improper for the final purpose of the work
(3D printing).
Thus, agglomerates should be avoided, always seeking dispersed suspensions that, in addition to
having a lower viscosity, ensure the homogeneity of the paste. For this reason, it is necessary that the
dispersant involves all the particles to avoid sedimentation and consequent segregation of phases.
Through the evaporation method, the concentration of solids may be increased allowing the paste to
remain homogeneous and stable for a longer period of time. In fact, the initial low concentrations of
solids allowed Dolapix CE64 to interact with all the ceramic particles of the fluid suspension, achieving
a homogenous distribution, and enable an easy agitation of the suspension, which contributed to the
breaking of eventual small aggregates.
Figure 35. Heterogeneos paste resulting from directly mixing 90% solids.
Figure 36. Homogeneous paste based on the evaporation of water.
44
3.1.1.3 Effect of the solids content
Zirconia based pastes with 1% Dolapix were prepared with different solid contents, by evaporation of a
starting paste (ink) with 60% of zirconia. Figure 37 shows the relative viscosity vs. shear rate for the
obtained suspensions over a range of solids concentrations between 70 and 92 wt%.
Figure 37. Viscosity as a function of the shear rate for different solids contents in pastes with 1%
Dolapix.
As might be expected, at low shear rates, viscosity was found to increase with increasing solids loading,
thus becoming more suitable to the purpose of the work. The decay in viscosity is exponential. Equation
16 was again applied, and the values obtained for consistency and flow rate are shown in Figure 38 and
39.
0
5
10
15
20
25
30
35
40
45
50
0 20 40 60 80 100 120 140 160
Vis
cosi
ty (
Pa.
s)
Shear Rate (1/s)
70%
80%
90%
91%
92%
pH 8
Dispersant dose: 1% of the solids
content
Solids loading, wt%
45
Figure 38. Average of the consistency for the pastes with different solids loadings and 1% Dolapix.
Figure 39. Average of the flow rate for the pastes with different solids loadings and with 1% Dolapix.
These results show that as the solids loading increases, the consistency of the paste also increases,
presenting relatively higher values of consistency for solids loading greater than 90% (70.91 ± 1.83,
82.61 ± 1.49 and 88.26 ± 1.62 Pa.s for 90, 91 and 92% respectively). In addition, the flow rate tends to
be much lower for zirconia suspensions with higher solids content, which demonstrates that pastes with
>90% solids loading are more suitable for the purpose of this study. It should be stressed that, in section
3.1.1., the objective was to find a concentration of Dolapix that allowed to produce an ink with the
minimum consistency and flow rate, to be used as starting point to obtain pastes with higher content of
solids, through an evaporation process. However, here, the objective is different: it is aimed to obtain
pastes with maximum consistency and minimum flow rate to enhance the printing process.
The viscosity was represented in function of the solids loading, at the same shear rates used previously
(see Figure 30). As expected, in all cases the viscosity increases with the solid content. More, higher
0
10
20
30
40
50
60
70
80
90
100
70% 80% 90% 91% 92%
Co
nsi
sten
cy (
Pa.
sn)
Solids Content (%)
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,45
0,5
70% 80% 90% 91% 92%
Flo
w r
ate
Solids Content (%)
46
shear rates lead to lower viscosities. It shall be underlined that all the studied pastes present a
pseudoplastic behavior, as is recommended, to facilitate the printing process (see section 3.2.1.2).
Figure 40. Viscosity versus solids loading behavior for zirconia slurries at 1 wt% Dolapix, at three
different shear rates.
Taking into account the obtained results, further studies were carried with pastes containing 90%, 91%
and 92% of solids.
3.1.2. Paste B
The results of the rheological measurements for paste B are shown in Figure 41.
Figure 41. Viscosity as a function of the shear rate for Paste B (4 essays).
0
5
10
15
20
25
30
35
40
45
50
60 65 70 75 80 85 90 95
Vis
cosi
ty (
Pa.
s)
Solids Loading (wt%)
2.653 1/s
4.393 1/s
7.275 1/s
19.95 1/s
33.04 1/s
0
5000
10000
15000
20000
25000
30000
0 5 10 15 20 25
Vis
cosi
ty (
Pa.
s)
Shear Rate (1/s)
a
b
c
d
Shear rate
47
It can be seen from Figure 41 that the viscosity values for the paste B at low shear rates are quite
superior to those found in the optimization process of Paste A. This means there is a great resistance
to flow at low shear rates. The logarithmitization of the results led to curves with three distinct zones
(Figure 42).
Figure 42. Logarithmic scale of viscosity as a function of shear rate.
A first reofluidifying zone, with a lower slope for low rates of deformation (1) another reofluidifying zone
with a more accentuated slope (2) and a smaller zone that gives an idea of being a Newtonian plateau
(3) were observed. It was not possible to apply the power law (Equation 14) to the reofluidifying zone,
to determine the rheological parameters of the paste, since the value of n (flow rate) came negative,
which does not have any physical meaning.
This problem could be overcomed by fitting other rheological models (e.g., Cross or Casson) [46], that
would imply a deeper analysis of the results, which is outside of the scope of this thesis.
3.2 Production of the specimens
Figures 43 and 44 represent the simulation of the robocasting extrusion of Paste A and and of Paste B,
respectively, using a syringe system. In the first case, Pastes with 90%, 91% and 92% of solids were
tested. However, due to the similarity of the obtained specimens, only images of 90% are shown.
0,0001
0,001
0,01
0,1
1
10
100
1000
10000
100000
0,1 1 10 100
Log
[Vis
cosi
ty (
Pa.
s)]
Log [Shear Rate (1/s)]
a
b
c
d
(1)
(2)
(3)
48
Figure 43. Extrusion of Paste A. A sintered sample is also shown.
Figure 44. Extrusion of Paste B. Sintered samples are also shown.
The requirements of the pastes are that it must flow with modest pressure through the syringe, retain
the desired shape after deposition, dry with minimal shape change after the printing simulation, and
sinter to high density. [31] Apparently, both pastes seem to fit these requirements (results on density
will be shown and discussed below).
For the extrusion of Paste B, a greater (manual) pressure was required compared to the one used in
Paste A, since the consistency of the first one is higher. Despite both pastes were able to be extruded,
form fine filaments and support multiple layers, their use in a 3D printer must take into account the
specifications of the equipment, since factors such as printing speed, pressure applied on the paste and
injector diameter may be critical for the quality of the obtained product.
For comparison purposes, specimens were also produced by molding (Figure 20).
All the specimens were sintered at 1450ºC.
Sintered
Sintered
49
3.4 Caracterization of the specimens
3.4.1 Density measurements
The relative density of the sintered samples (unpolished) produced by manual printing with Paste A and
Paste B (considering as reference a typical density value of 6.04 g/cm3 [54] for yttria zirconia) was
determined and is shown in Table 12. For comparison purposes, the density of a commercial sample of
yttria zirconia used in labs of dental prosthetics, was also measured.
Table 12. Average relative density of specimens obtained with Paste A (with different content of
solids), Paste B and commercial yttria zirconia.
90 91 92 Paste B Sample C
Relative Density
90.3% 93.2% 96.6% 83.8% 99.3%
For Paste A, all relative density values were above 90%. As expected, the higher the amount the solids,
the higher the density. Despite this, these values could still be optimized, for example with better
degassing of the paste to prevent pore formation.
In contrast, the density of the specimens produced with Paste B was significantly lower (83.8%).
Therefore, it is expected the existence of more space between the particles (i.e., pores).
The relative density of commercial yttria zirconia samples was 99.3%, which is above the value reported
by the manufacturer (96 – 99%). [31]
From the results presented above, it may be concluded that samples produced with Paste A higher
content of solids are the most promising, since they present a relative density closer to that observed
for the commercial material.
3.4.2 Porosity analysis
Porosity was evaluated through an imaging technique for different polished specimens produced with
Paste A (90, 91 and 92%), and with paste B. The analysis was also performed for polished commercial
Zirconia. Figure 45 shows typical images obtained my optical microscopy for the studied samples. Red
areas correspond o the pores identified by ImageJ software.
50
Figure 45. (1–3) Paste A: 90%, 91% and 92%, respectively; (4) Paste B and (5) Commercial Zirconia.
In the three samples produced with paste A (90, 91 and 92%) it was observed the existence of pores
randomly distributed. The same is also observed for sample B However, in this case more pores were
observed. Existing pores may allow gases or liquids to penetrate the sample, reducing its mechanical
strength and eventually the useful life of the material. Pores were almost absent in commercial zirconia
(sample C).
Figure 46 presents the percentage of the porous area estimated by the used software. The porosity of
the samples of Paste A produced with 90% and 91% of solid content are quite similar, although the
values dispersion is relatively high. Samples produced with the higher content of solids (92%) were
those that presented the lower percentage of porosity. As expected, from the visual analysis of the
images, specimens of paste B were the more porous (porosity 6.06 ± 1.18 %), while samples of
commercial zirconia were the less porous (porosity 0.163 ± 0.07%).
In a general way, the porosity results corroborate the tendency found for relative density values: the
higher the porosity, the lower the relative density.
1 2 3
4 5
51
Figure 46. Porosity determined for the specimens produced with Paste A (for 90, 91 and 92% of solids
loading), Paste B and commercial zirconia, C.
The distribution of pore size was evaluated for the prepared samples. The pore distribution in sample C
is not presented since its porosity is very small.
Figure 47. Distribution of sizes of the pores. (1–3) Paste A: 90%, 91% and 92%, respectively and (4)
Paste B.
0
1
2
3
4
5
6
7
8
90% 91% 92% B C
Po
rosi
ty (
%A
rea)
1 2
3 4
52
According to the distribution of sizes of the pores presented in the histograms of Figure 47, it can be
observed that all the samples present a high number of pores with small dimensions, being prevalent
the pores with diameter lower than 4 µm. As expected from the simple visualization of the optical
microscopy images (Figure 45), samples prepared with Paste B are the ones that present the higher
number of pores. Among the samples prepared with Paste A, the correspondent to 92% of solids shows
a number of pores with dimensions <10 µm significantly lower than the others. More, in comparison with
this samples, those prepared with pastes containing 90% and 91% of solids present a higher dispersion
of pore sizes.
From the presented results, it was possible to conclude that the 92% is the one that leads to specimens
with higher density (close to the reference value) and lower porosity. In this way, Paste A with 92% of
solids was chosen as the best one to produce specimens and, consequently, to continue this study.
3.4.4 Surface Morphology/Topography
The surface morphology of samples produced with Paste A with 92% of solids and Paste B was analyzed
by SEM. Figure 48 and 49 show typical images obtained for these samples. For comparison purposes,
an image of commercial zirconia is also presented (Figure 50).
Figure 48. SEM image of sample A.
53
Figure 49. SEM images of sample B.
Figure 50. SEM image of commercial zirconia (C).
B1
B2
54
As observed through the SEM images, sample B is more porous than sample A, in agreement with
optical microscopy observation. However, the distribution of pores is not homogeneous: there are places
with a high density of pores (Figure 49B1) and others where this is lower (Figure 49B2). Sample C
presents a quite uniform surface, similar to that found for Sample A.
The surface topography of the samples between pores was studied by AFM. The following images
(Figures 51, 52 and 53) depict the surface of the samples.
Figure 51. AFM image of the surface area of Sample A, and the respective 3D profile of the surface
sample.
Figure 52. AFM image of the surface area of Sample B, and the respective 3D profile of the surface
sample.
110.00 nm
0.00 nm
4.0µm
110.00 nm
0.00 nm
110.00 nm
0.00 nm
4.0µm
110.00 nm
0.00 nm
55
Figure 53. AFM images of the surface area of comercial zirconia sample, and the respective 3D profile
of the surface sample.
The topography of the samples A and B are quite similar: some scratches are visible in the surface with
a random orientation. In contrast sample C presents a quite different surface: no scratches were
observed on the surface, but a granular/globular morphology can be identified. This may be due to the
pull out of zirconia grains during the polishing process.
Roughness of intra-oral hard surfaces as zirconia dental restorations have impact on the adhesion and
the retention of oral microorganisms. At surface irregularities and asperities (like cracks, porous, and
other stagnant sites), bacteria, once attached, can survive longer because they are protected against
natural removal forces and also against oral hygiene measures, like toothbrushing. [74] Smooth surfaces
(with lower roughness) tend to minimize dental plaque formation, thereby decreasing the occurrence of
caries and other dental diseases. [75] Thus, the demand of dental prosthesis with lower surface
roughness has been focus of clinical attention.
Figure 54 depicts the average roughness (Ra) determined from a set of AFM images (20µm2 x 20 µm2)
for the surface of zirconia samples.
Figure 54. Zirconia mean roughness (Ra).
1.50 µm
0.00 µm
4.0µm
1.81 µm
0.00 µm
0
20
40
60
80
100
120
140
A B C
Ra
(nm
)
56
Samples A and B present values of Ra similar among each other but significantly lower than those found
for sample C (about 13 times lower). It must be stressed that these measurements were carried out in
inter-pores zone, and thus the porosity does not affect the obtained values. Furthermore, the roughness
depends on the material’s microstructure which is determined by the grain size. According to the
manufacturer the commercial sample (C) presents a grain size of 850 nm. For sample A and B a lower
grain size would be expected, since they were prepared with a zirconia powder whose particles had 150
nm and whose sintering temperature was low (1450 ºC), avoiding the grain growth. [76], [77]
3.4.3 Wettability tests
Wettability was studied for all samples, A, B and C, by measuring the contact angle through the sessile
drop technique. Figure 55 shows the obtained water contact angles measured for the different
specimens.
Figure 55. Contact angle for zirconia samples.
Despite the existence of a deviation in the values, all samples had a mean contact angle lower than 90°,
which means that a hydrophilic characteristic was maintained in all samples. The same also happens in
enamel, which has a contact angle below 60°. [74], [78]
Sample C led to the highest contact angle, 66 ± 10º. Samples A and B present lower and closer values
among each other (44 ± 5º and 36 ± 6º, respectively).
Hydrophilicity is an important parameter for dental prosthesis, playing a relevant role in the adhesion of
microorganisms and biomacromolecules existent in the oral environment. It also affects the comfort
provided by the prosthesis. As referred previously, wettability depends on the chemical and physical
characteristics of the surface. In the present case, all surfaces are chemically similar, since the samples
0
10
20
30
40
50
60
70
80
90
A B C
Co
nta
ct A
ngl
e (D
egre
es)
57
are made of zirconia-based materials. Differences in roughness may explain the different wettability
behavior. [79]
Although in previous studies [80] it has been hypothesized, that surface roughness may improve
wettability by reducing the contact angle, the wettability behavior in this study was inverse: the surfaces
more hydrophilic were the smoothest, A and B.
Wenzel (1936) formulated a model to predict the variation of wettability with roughness. The Wenzel
model [81] assumes that the liquid establishes contact with the entire rough surface, filling its
depressions (Figure 56A). According to the Wenzel's theory:
𝐶𝑜𝑠(Ɵ𝐴) = 𝑟𝑐𝑜𝑠Ɵ (𝟏𝟖)
where ƟA is an apparent contact angle and r is the ratio of the real rough surface area to the projected
perfectly smooth surface. Since the parameter r is always greater than 1, this model predicts that the
contact angle observed when a liquid wets the surface (Ɵ<90º) will decrease when the surface becomes
more rough (ƟA <Ɵ).
In turn, the Cassie-Baxter model (1944) [81] assumes that the liquid does not completely wet the rough
surface, due to the air remaining between the depressions of the surface (Figure 56B), being the contact
angle measured on a rough surface given by Equation 19:
𝐶𝑜𝑠(Ɵ𝐴) = −1 + 𝜙𝐿𝑆[𝑐𝑜𝑠Ɵ + 1] (𝟏𝟗)
where 𝜙𝐿𝑆 is the fraction of the liquid-solid interface (hence 𝜙𝐿𝑆 −1 is the fraction of the liquid-air
interface).
Figure 56. Wenzel (a) and Cassie-Baxter (b) models.
The wettability results obtained in this study seem to be in agreement with the Cassie-Baxter model: the
high roughness of samples C shall lead to the formation of air pockets that increase the water contact
angle.
58
It should be stressed that a higher surface roughness its generally associated to a higher tendency to
promote bacterial plaque formation. [75] However, several studies referred that bacterial adhesion is
favored by the increase of hydrophilicity. [74] In this case, the two factors (roughness and hydrophilicity)
act in an opposite way concerning the bacterial adhesion. Studies with microorganisms would have to
be performed to clarify this issue. Different bacterial species commonly found in mouth should be used,
since it is known that their behavior may differ significantly. [82], [83]
3.4.5 Tribological behavior
Friction and wear are of most importance for the performance of prosthetic materials. Wear tests were
performed in to mimetized the local contact of zirconia against zirconia using artificial saliva as lubricant
and a load within those found in the mastication process (see section 2.2.4.5). Wear tracks were sought
through AFM, but none were found in any of the three samples. This means that the load used does not
induce the wear of the tested materials, and that the produced specimens (A and B) offer a wear
resistance as good as the commercial sample (C). However, differences were observed concerning the
friction coefficient (Figure 57).
Figure 57. Friction coefficient for zirconia samples.
The friction coefficient depends on the surface energy (adhesion component) and also on the surface
roughness (plow component). [67] The obtained results show that the coefficient of friction follows the
same trend observed for the roughness, being the highest value achieved for sample C. A higher
roughness favors the increase of the friction coefficient. On the other hand, although all surfaces are
hydrophilic, a higher contact angle (lower wettability) shall impair the lubrication, increasing the friction
coefficient. This means that samples A and B present a better tribological behavior than sample C.
0
0,05
0,1
0,15
0,2
0,25
0,3
A B C
Fric
tio
n C
oef
fici
ent
59
Unlubricated sliding tests already performed on Y2O3 doped in zirconia ceramics showed that the friction
coefficients varied in the range of 0.5–1. [84], [85] No data was found concerning lubricating systems.
The obtained results show that the use of saliva as a lubricant shall contribute to the lower values found
for the friction coefficients of the three samples (0.12–0.25), due to the improvement of the lubrication
conditions.
3.4.6 Microhardness tests
The Vickers hardness of the studied materials was measured for different loads. The obtained results
are presented in Figure 58.
Figure 58. Zirconia microhardness for each sample.
Considering that the reported hardness for yttrium-zirconia is approximately 1250-1300HV [12], [86] it
can be seen that the experimental values obtained for the tested samples are close to this reference,
independently of the applied load.
No clear effect was observed concerning the applied load: while for samples prepared with Paste A,
higher loads lead to lower average hardness values, for samples prepared with Paste B the increase of
load results in higher values for hardness and for commercial samples the hardness values are not
affected by the load. It must be underlined that observed variations can be due to the heterogeneity,
taking into account the error associated to the measurement. The similarity between the obtained values
for the three types of samples allow to conclude that despite their different initial compositions, samples
A and B present suitable characteristic in terms of hardness.
0
200
400
600
800
1000
1200
1400
1600
A B C
Mic
roh
ard
nes
s A
vera
ge (
HV
)
1kg
10kg
30kg
60
Understanding the fracture behavior is also essential for the development of "tooth-like" restorative
materials. For that, fracture toughness was estimated using a calculation method (Ki,c) that involved
measurements of the lengths of the cracks, which emanate from the corners of Vickers indentation
diagonals. The SEM images presented in Figure 59 illustrates this procedure. As referred in section
2.2.4.6, the calculation also requires the knowledge of the material hardness and of its elasticity
modulus.
Figure 59. SEM image of an indentation in the commercial sample (C).
Three different equations reported in literature [68] were used to calculate the toughness (Equations 10,
11 and 12, section 2.2.4.6). The microhardness values used were the experimental values for 30 kg
(1139.5 HV, 1353 HV and 1297.5 HV for A, B and C, respectively). The obtained results were plotted in
Figure 60.
c1, length
61
Figure 60. Fracture toughness of samples A, B and C, with the use of different equations.
It was observed that the use of different equations led to different toughness values. However, in
qualitative terms the tendency was kept in all cases: the highest value was observed for samples B,
followed by C and finally A. The obtained average values, varied in a range of 3–6.5 Pa⋅m1/2, and are
lower to those reported for yttria zirconia fracture toughness, 8–10 Pa⋅m1/2 [54]. However, they are within
the range observed for dental ceramics used in dental restoration (see Table 2).
0
1
2
3
4
5
6
7
8
9
10
Equation 10 Equation 11 Equation 12
Kc,
i (P
a⋅m
1/2)
A
B
C
62
Chapter 4 – Conclusions
Zirconia-based pastes containing a dispersant (Dolapix CE64) were formulated and optimized to be
used in the production of dental parts by robocasting, based on the study of their rheological behavior.
It was concluded that:
1. A concentration of 1% of Dolapix on the solids content allowed to obtain minimum values of
consistency and flow rate, in suspensions with 60% solids.
2. Contrarily to what happened with the direct preparation of the pastes with high solids content,
the preparation of suspensions based on the evaporation of water (60% to 90% solids loading)
resulted in homogeneous pastes.
3. Concentrations > 90% of solids led to the higher values of viscosity.
Taking into account the previous findings, specimens prepared with pastes containing 90%, 91% and
92% of solids and 1% of Dolapix (type A) were characterized and compared with samples obtained from
additivated zirconia based-paste provided by CTCV (type B). It was found that:
4. The manual printability of pastes Type A was superior to that of paste B, since the former could
pass throw thinner orifices.
5. The increase in the solids content led to an increase of density of the type A materials. B
samples presented a lower density than A.
6. From all the tested samples, the one with 92% solids showed the lower porosity, being the pores
diameter generally lower than 4 µm.
Comparison with commercial zirconia samples (type C) showed that type A samples with 92% solids
are the most promising, concerning density and porosity. Thus, they were selected for further
characterization, in comparison with samples of type B and C. The following properties were evaluated:
topography/morphology, wettability, tribological behavior, microhardness and toughness. It was
observed that:
7. Samples of type A presented the lower roughness (7 ± 1 nm).
8. The water contact angle of samples A and B was significantly lower than that found for samples
C, being of the order of 40º.
9. No wear was detected in any of the samples, under loading conditions similar to those observed
in the mouth. Samples A led to the lower friction coefficient.
10. The microhardness of all the tested samples falls within the range of values reported for yttria-
zirconia commercial samples.
11. The highest values of toughness were obtained for samples B and C. However, all the values
are within the typical range found for ceramic dental materials.
Overall, the results showed that specimens produced with Paste A containing 92% solids and 1%
Dolapix, presented adequate properties for dental applications. Taking into account the preliminary test
of printability, the referred paste may be used for production of dental parts by robocasting.
63
Chapter 5 – Future work
The work on the optimization of zirconia-based pastes for dental prosthesis through robocasting is still
ongoing as part of a project which is running in parallel with the study presented in this thesis. In this
way, there are several aspects of research regarding robocasting applied to dental restorations, that
must be addressed such as:
1. Optimization of the different printing parameters such as the nozzle diameter, the chosen shear
rate and the injection’s velocity.
2. Study other properties of the obtained final products (e.g., flexural strength and wear resistance
using chewing simulation tests).
3. Study of the microorganisms adhesion behavior onto the produced materials.
4. Study of the protein adsorption onto the produced materials.
5. Evaluation of the cytotoxicity of the produced materials.
64
References
[1] Z. R. Zhou and J. Zheng, “Tribology of dental materials: a review,” J. Phys. D. Appl. Phys., vol.
41, no. 11, p. 113001, 2008.
[2] E. Atzeni and A. Salmi, “Evaluation of additive manufacturing (AM) techniques for the production
of metal-ceramic dental restorations,” J. Manuf. Process., vol. 20, pp. 40–45, 2015.
[3] S. Roy and B. Basu, “Mechanical and tribological characterization of human tooth,” Mater.
Charact., vol. 59, no. 6, pp. 747–756, 2008.
[4] P. Gupta and M. R. Sankar, “Advanced Materials State of Art on Tribomechanical andTribo
chemical Wear of Human Dental Enamel,” vol. 3, no. 1, pp. 119–126, 2013.
[5] M. A. Garrido, I. Giráldez, L. Ceballos, M. T. G. del Río, and J. Rodríguez, “Nanotribological
behaviour of tooth enamel rod affected by bleaching treatment,” Wear, vol. 271, no. 9–10, pp.
2334–2339, 2011.
[6] A. A. Madfa and X. G. Yue, “Dental prostheses mimic the natural enamel behavior under
functional loading: A review article,” Jpn. Dent. Sci. Rev., vol. 52, no. 1, pp. 2–13, 2016.
[7] A. Branco, A. P. Valagão, and C. Figueiredo-Pina, “Study of the effect of pH and of the presence
of remineralizing agents on teeth whitening treatments with doped mixtures based on hydrogen
peroxide,” pp. 1–31.
[8] G. Balooch, G. W. Marshall, S. J. Marshall, O. L. Warren, S. A. S. Asif, and M. Balooch,
“Evaluation of a new modulus mapping technique to investigate microstructural features of
human teeth,” J. Biomech., vol. 37, no. 8, pp. 1223–1232, 2004.
[9] V. Imbeni, J. J. Kruzic, G. W. Marshall, S. J. Marshall, and R. O. Ritchie, “The dentin–enamel
junction and the fracture of human teeth,” Nat. Mater., vol. 4, no. 3, pp. 229–232, 2005.
[10] H. H. K. H. Xu et al., “Indentation Damage and Mechanical Properties of Human Enamel and
Dentin,” J. Dent. Res., vol. 77, no. 3, pp. 472–480, 1998.
[11] T. K. James Zhijian Shen, “Advanced Ceramics for Dentistry,” 2014. [Online]. Available:
https://books.google.pt/books?id=Y-
9jwrHs0WEC&pg=PR13&lpg=PR13&dq=Advanced+Ceramics+for+Dentistry&source=bl&ots=-
QTGTJkfne&sig=0qrwzxOOGB1DnFZkwGQ75x8fYFg&hl=pt-
PT&sa=X&ved=0ahUKEwi6jfTjqKLXAhVEchQKHcTCDqgQ6AEIYjAI#v=onepage&q=Advanced
Ceramics for Dentis. [Accessed: 07-Nov-2017].
[12] F. A. T. Rodrigues, “Efeito do branqueamento dentário com peróxido de hidrogénio na
morfologia , hidrofilicidade , propriedades mecânicas e tribológicas do esmalte,” pp. 1–54, 2015.
65
[13] S. D. Heintze, G. Zappini, and V. Rousson, “Wear of ten dental restorative materials in five wear
simulators--results of a round robin test.,” Dent. Mater., vol. 21, no. 4, pp. 304–17, 2005.
[14] R. Lewis and R. S. Dwyer-Joyce, “Wear of human teeth: a tribological perspective,” Proc. I MECH
E Part J J. Eng. Tribol., vol. 219, no. 1, pp. 2–19, 2005.
[15] K. Mundhe, V. Jain, G. Pruthi, and N. Shah, “Clinical study to evaluate the wear of natural enamel
antagonist to zirconia and metal ceramic crowns,” J. Prosthet. Dent., vol. 114, no. 3, pp. 358–
363, 2015.
[16] L. H. Mair, T. A. Stolarski, R. W. Vowles, and C. H. Lloyd, “Wear: mechanisms, manifestations
and measurement. Report of a workshop,” J. Dent., vol. 24, no. 1–2, pp. 141–148, 1996.
[17] E. Sajewicz, “Effect of saliva viscosity on tribological behaviour of tooth enamel,” Tribol. Int., vol.
42, no. 2, pp. 327–332, 2009.
[18] R. A. Gittens et al., “A review on the wettability of dental implant surfaces II: Biological and clinical
aspects,” Acta Biomater., vol. 10, no. 7, pp. 2907–2918, 2014.
[19] L. Y. Chen, Q. Q. Fang, W. Z. Jiang, X. P. Liao, J. Y. Ye, and H. Le Jin, “Study of the Hydrophobic
Properties of the Surface of an Enamel Material,” Mater. Sci. Forum, vol. 789, pp. 178–182,
2014.
[20] H. J. Busscher, “Modern Approaches to Wettability: Theory and Applications.” [Online]. Available:
https://books.google.pt/books?id=FhsGCAAAQBAJ&pg=PA249&lpg=PA249&dq=Wettability+of
+Surfaces+in+the+Oral+Cavity&source=bl&ots=rcrC7ao6xI&sig=XgcPY_3ySYoM6CWYjhlrIF_
7MnA&hl=pt-
PT&sa=X&ved=0ahUKEwjdnrmdzazXAhVDvhQKHSozDnsQ6AEIQTAD#v=onepage&q=Wetta
bility of. [Accessed: 07-Nov-2017].
[21] K. J. Kubiak, M. C. T. Wilson, T. G. Mathia, and P. Carval, “Wettability versus roughness of
engineering surfaces,” in Wear, 2011, pp. 523–528.
[22] J. Pihlaja, “Treatment Outcome of Zirconia Single Crowns and Fixed Dental Prostheses,”
University of Oulu Graduate School, Doctoral dissertation, 2016.
[23] E. Homaei, K. Farhangdoost, E. H. N. Pow, J. P. Matinlinna, M. Akbari, and J. K. H. Tsoi, “Fatigue
resistance of monolithic CAD/CAM ceramic crowns on human premolars,” Ceram. Int., vol. 42,
no. 14, pp. 15709–15717, 2016.
[24] “Dental Crowns.” [Online]. Available:
http://fgdentalcare.com/dentalcrowns_bridges_venears.php. [Accessed: 23-Apr-2017].
[25] Usha, G, T. . Prashanth, R. N. Roopa, G. Yashwanth, and Murtuza, “Advanced Ceramics a
Review of Material Science,” Journal of Dental Sciences and Research, vol. 5, no. 2. pp. 5–11,
66
2014.
[26] Invibio Biomaterials Solutions and C. a D. Cam, “New Material Options for Innovation in
Restorative and Prosthetic Dentistry,” pp. 1–8, 2011.
[27] J. Chevalier, What future for zirconia as a biomaterial?, vol. 27, no. 4. Elsevier Ltd., 2006.
[28] I. Denry and J. R. Kelly, “State of the art of zirconia for dental applications,” Dent. Mater., vol. 24,
no. 3, pp. 299–307, 2008.
[29] P. Pittayachawan, “Comparative study of physical properties of zirconia based dental ceramics,”
Dr. thesis, UCL (University Coll. London)., 2009.
[30] E. E. Daou, “The zirconia ceramic: strengths and weaknesses.,” Open Dent. J., vol. 8, pp. 33–
42, 2014.
[31] “ZirCad e.max.” [Online]. Available: http://www.ivoclarvivadent.com/en/p/all/products/all-
ceramics/ips-emax-technicians/ips-emax-zircad. [Accessed: 11-Nov-2017].
[32] R. Van Noort, “The future of dental devices is digital,” Dent. Mater., vol. 28, no. 1, pp. 3–12, 2012.
[33] V. T. Le, H. Paris, and G. Mandil, “Process planning for combined additive and subtractive
manufacturing technologies in a remanufacturing context,” J. Manuf. Syst., vol. 44, pp. 243–254,
2017.
[34] E. J. Bae, I. Do Jeong, W. C. Kim, and J. H. Kim, “A comparative study of additive and subtractive
manufacturing for dental restorations,” J. Prosthet. Dent., vol. 118, no. 2, pp. 187–193, 2017.
[35] N. Guo and M. C. Leu, “Additive manufacturing: Technology, applications and research needs,”
Front. Mech. Eng., vol. 8, no. 3, pp. 215–243, 2013.
[36] D. Drummer, C. Williams, and J. Parthasarathy, “Additive Manufacturing: Innovations, Advances,
and Applications,” no. 15, pp. 389–401, 2013.
[37] A. Ambrosi and M. Pumera, “3D-printing technologies for electrochemical applications,” Chem.
Soc. Rev., vol. 45, no. 10, pp. 2740–2755, 2016.
[38] D. S. Reis, “Aplicação de alumina em robocasting,” 2017.
[39] M. Leary et al., “Additive manufacture of custom radiation dosimetry phantoms: An automated
method compatible with commercial polymer 3D printers,” Mater. Des., vol. 86, pp. 487–499,
2015.
[40] H. N. Mai, K. B. Lee, and D. H. Lee, “Fit of interim crowns fabricated using photopolymer-jetting
3D printing,” J. Prosthet. Dent., vol. 118, no. 2, pp. 208–215, 2017.
[41] M. Faes, H. Valkenaers, F. Vogeler, J. Vleugels, and E. Ferraris, “Extrusion-based 3D printing
67
of ceramic components,” Procedia CIRP, vol. 28, pp. 76–81, 2015.
[42] J. A. Lewis, J. E. Smay, J. Stuecker, and J. Cesarano, “Direct ink writing of three-dimensional
ceramic structures,” J. Am. Ceram. Soc., vol. 89, no. 12, pp. 3599–3609, 2006.
[43] E. Feilden, E. G. T. Blanca, F. Giuliani, E. Saiz, and L. Vandeperre, “Robocasting of structural
ceramic parts with hydrogel inks,” J. Eur. Ceram. Soc., vol. 36, no. 10, pp. 2525–2533, 2016.
[44] J. Cesarano, “A Review of Robocasting Technology,” MRS Proc., vol. 542, pp. 133–139, 2011.
[45] N. R. F. A. Silva, L. Witek, P. G. Coelho, V. P. Thompson, E. D. Rekow, and J. Smay, “Additive
CAD/CAM process for dental prostheses,” J. Prosthodont., vol. 20, no. 2, pp. 93–96, 2011.
[46] T. Ii, “Reologia e Estrutura dos Alimentos Viscosidade de Fluidos Newtonianos e.”
[47] E. Peng et al., “Robocasting of dense yttria-stabilized zirconia structures,” J. Mater. Sci., vol. 53,
no. 1, pp. 1–27, 2017.
[48] S. P. Rao, S. S. Tripathy, and A. M. Raichur, “Dispersion studies of sub-micron zirconia using
Dolapix CE 64,” Colloids Surfaces A Physicochem. Eng. Asp., vol. 302, no. 1–3, pp. 553–558,
2007.
[49] E. Özkol, J. Ebert, K. Uibel, A. M. Wätjen, and R. Telle, “Development of high solid content
aqueous 3Y-TZP suspensions for direct inkjet printing using a thermal inkjet printer,” J. Eur.
Ceram. Soc., vol. 29, no. 3, pp. 403–409, 2009.
[50] M. Biswas and A. M. Raichur, “Dispersion of zirconia in the presence of dolapix PC75,” J.
Dispers. Sci. Technol., vol. 31, no. 9, pp. 1173–1177, 2010.
[51] J. G. da Silva, “REOLOGIA E MICROESTRUTURA NA ESTABILIZAÇÃO DE SUSPENSÕES
CONCENTRADAS DE Al2O3,” p. 134, 2003.
[52] P. Kohorst, M. P. Dittmer, L. Borchers, and M. Stiesch-Scholz, “Influence of cyclic fatigue in water
on the load-bearing capacity of dental bridges made of zirconia,” Acta Biomater., vol. 4, no. 5,
pp. 1440–1447, 2008.
[53] S. Zürcher and T. Graule, “Influence of dispersant structure on the rheological properties of
highly-concentrated zirconia dispersions,” J. Eur. Ceram. Soc., vol. 25, no. 6, pp. 863–873, 2005.
[54] “Yttria Zirconia.” [Online]. Available: http://www.zircomet.com/det/10951/Yttria-Stabilised-
Zirconium-Oxides/,. [Accessed: 26-Jul-2017].
[55] “Powdered Sugar as a Binder.” [Online]. Available:
https://www.google.com/patents/EP1430158A4?cl=en. [Accessed: 11-Nov-2017].
[56] “Modular Advanced Rheometer System,” Fisher Thermo Scientific. [Online]. Available:
http://www.thermo.com.cn/Resources/200802/productPDF_105.pdf. [Accessed: 03-Nov-2017].
68
[57] I. Mazali, “Determinação da densidade de sólidos pelo método de Arquimedes,”
http://lqes.iqm.unicamp.br/, 2010. [Online]. Available:
http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Determina??o+da+Densidade
+de+S?lidos+pelo+M?todo+de+Arquimedes#0. [Accessed: 10-Sep-2017].
[58] F. T. Rodrigues, A. P. Serro, M. Polido, A. Ramalho, and C. G. Figueiredo-Pina, “Effect of
bleaching teeth with hydrogen peroxide on the morphology, hydrophilicity, and mechanical and
tribological properties of the enamel,” Wear, vol. 374–375, pp. 21–28, 2017.
[59] A. Force and M. Afm, “Basic Theory,” Energy, vol. 67, no. 1, pp. 1–8, 1948.
[60] B. Hafner and B. Hafner, “Scanning Electron Microscopy Primer,” Cities, pp. 1–29, 2007.
[61] “Scanning Electron Microscope.” [Online]. Available: http://www.mee-inc.com/hamm/scanning-
electron-microscopy-sem/. [Accessed: 21-Oct-2017].
[62] C. G. Figueiredo-Pina et al., “Tribological behaviour of unveneered and veneered lithium
disilicate dental material,” J. Mech. Behav. Biomed. Mater., vol. 53, pp. 226–238, 2016.
[63] W. Abdallah et al., “Fundamentals of Wettability,” Oilf. Rev., pp. 44–61, 2007.
[64] N. Lawson, “Composite Dental Materials: Wear,” Ref. Modul. Mater. Sci. Mater. Eng., no.
October 2015, p. , 2016.
[65] D. Cristina, “PhD program in Advanced Materials and Processing Course Seminário 2016 –
3aEdição Seminar À Procura do Atrito Zero,” 2016.
[66] “Masticatory Force.” [Online]. Available: https://en.wikipedia.org/wiki/Masticatory_force.
[Accessed: 23-Oct-2017].
[67] C. G. Figueiredo-Pina et al., “Effect of feldspar porcelain coating upon the wear behavior of
zirconia dental crowns,” Wear, vol. 297, no. 1–2, pp. 872–877, 2013.
[68] F. Sergejev and M. Antonov, “Comparative study on indentation fracture toughness
measurements of cemented carbides,” Proc. Est. Acad. Sci. Eng, vol. 12, no. 4, pp. 388–398,
2006.
[69] P. a Dearnley, “A review of metallic, ceramic and surface-treated metals used for bearing
surfaces in human joint replacements.,” Proc. Inst. Mech. Eng. H., vol. 213, no. 2, pp. 107–135,
1999.
[70] F. Rickhey, K. P. Marimuthu, J. H. Lee, H. Lee, and J. H. Hahn, “Evaluation of the fracture
toughness of brittle hardening materials by Vickers indentation,” Eng. Fract. Mech., vol. 148, pp.
134–144, 2015.
[71] Zschimmer&Schwarz, “Mechanisms of action of deflocculants and dispersants in ceramic
69
bodies,” Zschimmer Schwarz GmbH Co KG, pp. 1–15, 2013.
[72] D. Y. Zirconia, “Electronic Supporting Information ( ESI ),” pp. 1–21.
[73] T. A. Baer, J. Cesarano III, P. Calvert, J. Cesarano, T. A. Baer, and P. Calvert, “Recent
developments in freeform fabrication of dense ceramics from slurry deposition,” Solid Free. Fabr.
Proceedings, Sept. 1997, pp. 25–32, 1997.
[74] M. Quirynen and C. M. L. Bollen, “The influence of surface roughness and surface‐free energy
on supra‐ and subgingival plaque formation in man: A review of the literature,” Journal of Clinical
Periodontology, vol. 22, no. 1. pp. 1–14, 1995.
[75] M. D. McConnell, Y. Liu, A. P. Nowak, S. Pilch, J. G. Masters, and R. J. Composto, “Bacterial
plaque retention on oral hard materials: Effect of surface roughness, surface composition, and
physisorbed polycarboxylate,” J. Biomed. Mater. Res. - Part A, vol. 92, no. 4, pp. 1518–1527,
2010.
[76] B. STAWARCZYK, A. EMSLANDER, M. ROOS, B. SENER, F. NOACK, and C. KEUL, “Zirconia
ceramics, their contrast ratio and grain size depending on sintering parameters,” Dent. Mater. J.,
vol. 33, no. 5, pp. 591–598, 2014.
[77] B.-N. KIM, T. S. SUZUKI, K. MORITA, H. YOSHIDA, J.-G. LI, and H. MATSUBARA, “Evaluation
of densification and grain-growth behavior during isothermal sintering of zirconia,” J. Ceram. Soc.
Japan, vol. 125, no. 4, pp. 357–363, 2017.
[78] R. M. A. E. G. H. Glantz, P.O. Baier, R.E. Attstrom, “Comparative clinical wettability of teeth and
intraoral mucosa,” J. Adhes. Sci. Technol., vol. 5, no. 5, pp. 401–408, 1991.
[79] A. Noro, M. Kaneko, I. Murata, and M. Yoshinari, “Influence of surface topography and surface
physicochemistry on wettability of zirconia (tetragonal zirconia polycrystal),” J. Biomed. Mater.
Res. - Part B Appl. Biomater., vol. 101 B, no. 2, pp. 355–363, 2013.
[80] W. M. Al-Omari, C. a Mitchell, and J. L. Cunningham, “Surface roughness and wettability of
enamel and dentine surfaces prepared with different dental burs.,” J. Oral Rehabil., vol. 28, pp.
645–650, 2001.
[81] G. Bracco and B. Holst, Surface science techniques, vol. 51, no. 1. 2013.
[82] H. K. Kuramitsu, X. He, R. Lux, M. H. Anderson, and W. Shi, “Interspecies Interactions within
Oral Microbial Communities,” Microbiol. Mol. Biol. Rev., vol. 71, no. 4, pp. 653–670, 2007.
[83] “Bacterial Mouth Infections.” [Online]. Available:
https://emedicine.medscape.com/article/1081424-overview. [Accessed: 20-Nov-2017].
[84] M. Woydt, J. Kadoori, K. H. Habig, and H. Hausner, “Unlubricated sliding behaviour of various
zirconia-based ceramics,” J. Eur. Ceram. Soc., vol. 7, no. 3, pp. 135–145, 1991.
70
[85] H. R. Pasaribu, J. W. Sloetjes, and D. J. Schipper, “Friction reduction by adding copper oxide
into alumina and zirconia ceramics,” Wear, vol. 255, no. 1–6, pp. 699–707, 2003.
[86] F. J. L. Alves, “Materiais de construção mecânica ii (em307),” vol. 1704, no. 1742, pp. 1–12,
2005.